What Causes A Heavy Menstrual Period?

I was experiencing pregnancy symptoms for 3 weeks, then started my period 2 weeks late and it only lasted 2 days and it was heavier than normal as I normally don t come on heavily, is there any explaination for this? Could it be possible I m pregnant or not? I only had unprotected sex once. I m still experiencing pregnancy symptoms now, I finished 2 days ago.