Well, he could still be feeling bad--it sounds like he was really sick and no one feels good with congestion. However, my sweet daughter was absolutely awful right around the time she turned 3. Her preschool teacher has years and years of experience, so she handled a classroom full of cranky 3-year-olds with ease and grace, but she would occasionally say it had been a rough day with all of the girls doing a little hair-pulling and screeching, so I know there were some really bad days at school (and I was relieved that it was not just my own child who was acting up).
Based on those episodes, I am guessing that all 3-year-olds have some really stinky phases, and you may be hitting one with your son. Good luck! I will add that 3 was one of my favorite ages as well, so you have some wonderful times ahead of you this year.