It sounds more likely that you are experiencing PMS symptoms and your period will be here soon.

Since you got a period after having sex, odds are you are not pregnant. Even if the amount of flow was lighter, it is really very rare for women to have what they mistake as an actual period when they are pregnant. Usually, the women this happens to have very light periods to start with. Even in my case, I did have very light periods--needing a tampon 1 day only and then just pantyliners thereafter. I'm 12 weeks pregnant, and I did have spotting around the time of my period, but even that I never mistook for my period because it was just spotting and it was mostly pink/brown in color instead of red and I never had actual flow.

If you are still worried and haven't had unprotected sex since the one encounter, you can take a home pregnancy test now since you would have gotten pregnant before your last period.