Why do I get butterflies every time I seem my ex, a picture or even when people say his name!?

its been 5 months since we broke up. He is dating a new girl now... and I think I still have feelings for him. I actually can't stand it. I'm not obsessed with him, and the fact that he's with another girl doesn't really bother me.. its just the fact that I don't even know him anymore, and he just makes me so mad because he's a total d*** to me, and doesn't even talk to me anymore because of his new girl. and pretty soon I have to go to the same school as them. I don't know how to deal with this. I'm under a lot of stress these days and I just want to hang out with him? it's weird. I think I just miss having someone there for me. it just doesn't feel like I will ever stop missing him.