If you body is very used to swallowing a bunch of bacteria, then that's it's normal state. Your white blood cells are working away and dealing with what is going down your throat into your body.
The minute you change that, your body is like...what the hell? I'm really used to working hard at clearing all this stuff up constantly...where is it?
Then your white blood cells go on a cruise to the Bahamas, and you sort of miss them doing their thing. After all, they've been working really hard, and that's exactly what you would expect. A "normal" state, that is changing rapidly.
It's OK though, I'm sure they will send you a postcard.