I am not a vet. I hope Meghane weighs in here. However, diagnosing online is obviously unreliable.
I think that could possibly be a seizure, or stroke? Obviously, you need a competent vet to examine him. Please let us know.

I have no clue as to what may have happened.

When my dog had seizures he would come to me, fall on the ground , his body would look ridged then he started to tremor and shake for about 1-3 minutes. It looked like he was being electricuted. After the seizure he would get up and pace the house for a good 15 minutes like he had pent up energy. After the episode he was back to normal.

Weakness and a dazed expression could mean he came close to fainting from a change in blood pressure or a rapid heartbeat associated with the run up the stairs. Was he panting heavily?

It could also have been a petit mal (minor) seizure (those do usually include spasms or twitching). Such spells can also be associated with hyperthyroidism.

Always a good idea to get a vet check.

It could have been. From what I know (which could be wrong) epilepsy usually shows up by or around 2 years old.

Although I am wondering - based on my own experience with my dog - is it possible your dog tooted and got more than he bargained for?? Which would explain him trying not to move and sitting quickly and acting funky?

Our bigger dog is pretty gassy, and on the rare occasion it ends up being more than that. And he acts pretty off when it happens. He tucks the tail too, he'll try to sit, then stand, then sit and stand, and his ears go back and he looks panicked. And he doesn't move normally until whatever was half way out, comes all the way out. He moves slowly and awkwardly. And he gets that sad, dopey look he gets when he's in trouble, maybe because he knows he's not supposed to poop in the house.

I would definitely have him checked out just to be safe, but if he comes back clean, he could've just had an oops.

I'm hoping it was just an oops moment, but he was so lethargic on the bed. He really looked unwell.

We've been keeping a close eye on him and he's been his usually bouncy self this weekend. He's going in for a bordetella booster on Thursday. I'll talk to them about it as well.

I'd hold off on the vaccine booster until you know for sure if he had a seizure or not. Ask your vet to do a titer instead if your worried about immunization levels. But under no circumstance would i allow a vaccine at this time.
In the meantime if you have a camcorder or anything else capable of taking a video keep it near by just in case there is another episode as it may help the vet very much if he/she sees the action taking place.

Another possibility is hypoglycemia. One of my dogs had that, and she would suddenly become very unsteady and "zoned out". At first it would pass quickly, but got progressively worse, and did resemble a type of seizure. No trembling though.

If he has more episodes, and other things have been ruled out, try giving him some honey when he's having an episode. Rub it on his gums/tongue. If he snaps out of it quickly after doing that, you'll want to have him tested for low blood sugar.

My dog was very old, and in her case, it turned out to be insulinoma. There are other causes of hypoglycemia though, so worth keeping in mind.

My dog is a 75# old english sheep/lab mix. Everytime I give her her heartworm med (this time Interceptor) she gets very sick. She is lethargic, won't hardly move, cries when she moves, won't eat, drink or go potty. I'm worried sick. Anyone else ever have this reaction to meds? Her pain with this really is weird but it always happens. She had been playing really hard the day before but she isn't out of shape. And we are travelling in our RV. Luckily Banfield Pet Hospitals have offices here. I've call a call into the vet.

I think seizures manifest themselves in different ways. Our Labrador "fell off the bed" two or three times; each time freaking himself (and us) out over the experience and he tried to readjust as soon as he could Those are my words for what happened. What REALLY happened, I think, is that our dog began having a seizure and as a result, fell off the bed! I think you need to look at your situation with an open mind and do what's best for your best friend. Seizures manifest in different ways -- talk to your vet -- go from there. We don't have medication for our dog -- we just know that it can happen occasionally and we're prepared to help him through it. Good luck!

Thank you all for your input. :) He's been his usual happy, active self the last few days; but we are still watching him carefully.

I've jotted down notes to take with me to the vet tomorrow and we'll run through everything. Our vet and his team are just great. I really trust them.

milliesgirl, if you check back please have a read of this information discussing dogs with sensitivity to Ivermectin (used in some heartworm meds).
http://www.vetmed.wsu.edu/depts-vcpl/
Or google ivermectin sensitivity

It's very serious and you should defiantly have a talk with a vet about your dogs reaction to heartworm meds.

Here is a great site about dogs and seizures. I have learned a lot just reading the doctor's site.
http://dogtorj.tripod.com/index.html
Darlene

The loss of bowel control and the sudden return to 'normal' ... I bet he had a very slight seizure. I'm not a vet. I have had two dogs with seizures though, and what you describe fits the descript.

My current dog with seizures does not usually have the stiff body/swimming legs symptoms; she just collapses, bounces back up, and returns to 'normal' very quickly. She's on potassium bromide and phenobarbital and in pretty good control these days.

If it happens again, and you can get a video (cell phone quality is good enough), that will help your vet out quite a lot.

With my border collie had an exercise induced collapse which is common for black labs and border collies. Their legs give way. Not enough blood flow.