Bleeding 7 days after period

My periods are always regular lasting 7 days and usually takes 26 days for me to start another period the next month but my last period only lasted for 4 days. I did have unprotected sex during and Now after 7 days of not having my period. I'm bleeding it's not a lot but it is like I'm getting ready to have my period again. It's very light but starting to get a little heavier and I'm getting cramping every so often and have had the symptoms that I normally get before starting my regular periods. I took at pregnancy test it was negative I'm hoping it stays that way but what could be the issue?