I’ve never felt this way before

So a little back story my brother is 26 and I’m 18 his best friend is 28. He’s known me since I was born and that would make him 10 and honestly I didn’t expect this at all I’ve known him (my brothers best friend) forever but I don’t know I can’t get him out of my head. It’s been this way for months and finally I majorly hinted it and it worked he caught on and it turns out he feels the same way. We both don’t want a serious relationship just sex so it works perfectly and honestly I don’t know why I feel this way I don’t want him to be my boyfriend but it’s like anytime I think about seeing him I get nauseous because I’m so nervous and I’m just thinking about him all the time. I honestly just had to get this off my chest because if where going to do this nobody can find out.