I don't get it

OK. There's been something that's really been bugging me. Mr. Gall is a staff member at my res, and we've known each other for a while now. Near the end of last year, I've noticed he's been yelling alot more, and has been in a bad mood almost every day. He was soo cool, and always joking around, but now, it seems like he's totaly changing. Near the end of lastyear, me and him weren't getting along, and frankly, I change my oppinion about him. Before, I thought of him as someone who is cool, always joked around, and understanding, but now, he's so serious, seems like never joke around, and seems to have some sort of problem with me spasifically. What did Ido to him? And with what happened last week, we barely talk anymore, and I found myself avoiding him.
How did I manage to get on his bad side? I'm so confused!