Maybe I didn't thin the paint enough?

Ok, I was painting with my AB the other day and everything seemed to be fine (I was using Tamiya acrylic, by the way). Then while I was in the middle of painting, I noticed that the paint seemed to be getting darker and and the texture was more coarse. I knew something was wrong, so I stopped painting and cleaned the AB. After the paint was dry, I touched it and sure enough, it felt kind of like 400 grit sand paper. But what's weird is, it started out like normal paint; it went on smooth and looked a little wet, but then it changed to a rough, dry consistency. I've never had this happen before. Does anyone have any ideas why this may have happened (maybe I didn't thin it enough)?