Never had sex, but get frisky during PMS?

My boyfriend and I have been together for 2 years (tomorrow 😉) and we've never had sex. We made an agreement to refrain from sex until we are married, but I get SO "horny" (more so frisky) while I'm PMSing. Why is this? Earlier today he tried on a white button up shirt and looked so good that I had to bite my finger from freaking out. I just wanted to pounce on him! I feel dirty for this, but does anyone know the real reason why this happens? It's like I can't help it!