I have an aerobic septic system, a couple of weeks ago I had a backup into the house. Called plumber out and he snaked the drain and unclogged it. Said there was a foreign object in there (inner sole from a shoe?) anyway...since then, I have noticed the water level in the back toilet is sometimes low, and there have been a few large bubbles come up when I flush it. Any ideas what is happening? Anything I can do without an assist from a plumber? I am a new home owner and clueless...so any help is greatly appreciated.

ps, my two toilets are back to back to one another, the bathrooms are side by side