E.K.
First of all, Great Job!!! My boys both decided on their 4th bdays that they were done with pullups, so my hat is off to you! Second, is there any chance that you could be pregnant? A few of my friends toddlers knew that they were pregnant before the mommies did and started changing some regular habits. I know it sounds really weird, but try it.
Other than that, how often do you change the reward he gets for 8 stickers? Maybe he's bored with it, and is treating it like a game. He may just need to see that you are serious about him staying potty trained.