I think I missed my period, what do?

Okay so I'm 17, and I think I've missed my period. It usually comes around the 23rd, sometimes the 21st or 25th, but now it's the 27th and I'm not even showing signs of it being on the way. I'm a virgin, but a couple weeks ago (can't remember exactly how many), my boyfriend and I were getting a little hot on my bed. We weren't exactly wearing a lot of clothes, and some pre-come or whatever got on my underwear (quite a bit, it soaked through). It was at the top, and I changed them just to be safe but I'm scared I might have gotten pregnant? I know this is probably irrational, but I can't think of any other reason I should have skipped seeing as I haven't changed my diet or lifestyle any. Please help me I'm really scared, is it even a possibility or am I just being silly?