Irregular Period And Getting Pregnant?

I've heard the stories about how it doesn't take a lot of semen to get a girl pregnant and it doesn't even have to be direct, (e.G. Outer lip of vagina). And that got me scared. A few weeks ago I had 'foreplay' with my boyfriend and if my memory served me right (i have terrible memory, even short-term), that night I think I rubbed some of his semen on my vagina lips by accident. I forgot it was on my fingers. I don't remember if I wiped it off or not, though. The thing is, i've always had irregular periods, but I suspect that my most recent is because of stress.. If that's even viable. Anyway, I haven't had a period in about 2 months. I'm pretty young so menopause is ruled out. Well, lately i've been getting some cramps. And I don't usually get menstual cramps, and they're starting to scare me. They're not really severe, but still scary. And i've been feeling a little weird and out of check lately. I'm scared to rule my lack of a period a reason for being pregnant because I haven't had one for awhile even before this whole thing happened. Can someone help me? Am I pregnant? How will I know? This is tearing me apart inside because i'm scared to death if I am.