Lighter Periods After Ejaculate On Stomach In The Earlier Cycle. Pregnancy Chance?

So you know, I am a hypochondriac and suffer from really bad anxiety , so I tend to panic over nothing, but I need to ask...a while ago one of my guy friends pressured me into letting him ejaculate on my stomach . Right afterwards I took a shower. I had my period at the time and had a tampon in. A month later, my period was on time, but considerably lighter. I am absolutely terrified that I could be pregnant, but I don t even know if it s possible. That s where I need your help. I m so scared. I am also a virgin , dont know if that makes a difference. Thanks.