I noticed it this morning. I had a cup of coffee and a bottle of water, and I had the taste after each one. I have sinus problems, and I wonder if that's the cause.
There's something in diet soda, if I drink it in the evening, the next morning I have a strange taste in my mouth. If this continues, keep track of what you eat and drink.
I've experienced that too--after I had diet soda once. I normally don't drink diet soda but it was the only kind around at the time. It has to do with the artificial sweetener as far as I know. I stay away from artificial sweeteners as much as possible.