Near September I became really good friends with a guy. Just friends mind you, and we talked a lot about personal things. Ex. I told him about my breakup a few months before. He told me about his problems too and that he didn't think he would be able to seek a relationship.
Which I don't really mind because, I don't think I was searching for a relationship: I was only searching for a friend.
Sure there were bad days and we had our minor disagreements, but mostly it was just teasing(friendly), understanding, talking and friendship.
He made me feel that I could trust people again after my breakup.
After a school trip (which I wasn't on), he suddenly started ignoring and avoiding ( this started a few days ago) me. And I have no clue why? I told him I'm sorry for whatever I did (still don't know what I did wrong) and then he just said I'm sorry you feel that way.
I feel like I did something to make him hate me but I can't think of anything...
I have only three friends that I truly trust and they keep me emotionally stable. He knows that he's one of them. The other two don't...
Do you have any suggestions of what to do? Or why he's suddenly like this? Or maybe if he'll be friends with me again?
I really miss him, selfish as it is...

If you really care about him and want him by your side as a friend, then you should go talk to him about it; ask what's happening and for an explanation since you deserve one. It's not selfish at all if you clearly do not know why he's suddenly like that, so you have every right to be straight-forward with him. There are various reasons of why he could possibly be avoiding you: maybe he misinterpreted something, heard something wrong, or it's just a personal issue of his (in this case, you can let him know that you are there for him in case he needs your help at all). In the end of it all, find out what it is that you should do, depending on the situation and how involved you are in it.