You said or did something that ticked off someone (for some asinine reason considering what I've seen of your posts) and they rated you down, which caused you to drop below a score of 3.01 (yes, yes, I know the numbers are no longer used/visible).
The stars come from having a certain number of posts. The only ones I know for sure are 1,000 posts is 3 stars, 5,000 posts is 4 stars and 10,000 posts is "There is no 5th star."