Boy Problems

Let me start by saying, i don't usually stress over guys. Like at all. I can think they're cute etc but i hardly crush. I'm a senior in hs and decide to take strong body despite being one of only two girls in that particular class. One day this guy, Tony, starts randomly talking to me even though i kept to myself. As time went on, we gradually started talking more and i noticed he'd stare at me sometimes and smile when i'd catch him. Whenever i walk up to the front to get water, he follows me and flirts a little (I think). We've been flirty for about two weeks now but he has a girlfriend. She's absolutely gorgeous and i would never try and break them up but ever since i started my period this month, all i want to do is have sex with him and be with him. I don't know if it's just the hormones but i've got it bad. Any advice?