not sure what to do

So my boyfriend is in the military and is home for Christmas. His ex has recently been going through a hard time so he started talking to her to help her out. Which I understand because I would most likely do that same. Yesterday he wanted to visit his school and see some of his old teachers. He couldn't find anyone to go with him so he went with her. He said nothing happened which I believe, but why out of all the people in the world did he choose her? He knows how I would feel about it. We have been fighting lately, but the night before we had talked about how we want to do anything to help make our relationship better. Then he goes and does this. I don't understand. I have been the most honest and loyal person while he's been away, I don't understand. I'm trying to forget about it but it's so hard. I don't know what to do.