What Causes An Early Menstrual Period?

hi! a couple of weeks ago i had sex while on my period. my partner pulled out, however i know there is still a chance of "precum". i also know it is very rare to get pregnant while on your period. however, i started my period today, 2 weeks early. it's just got me a little nervous. it seems like a very regular period, it looks regular, and my body feels like it normally does the first day of a period. the flow is mild, not light but not heavy. i was sick a week ago with an upper respiratory infection, and i know sometimes illness can trigger an early menstrual cycle. i'm 21 years old, in case age is a factor. i suppose i'm just wondering if it could be pregnancy, and if not then what is going on?!
Thank you,
concerned