What Causes Heavy Vaginal Bleeding And Stomach Cramps?

hi, i had what i thought was implantation bleeding ( light pink/brown) on the19th april, one week after i had unprotected sex. I took a pregnnacy test whichwas negative. However, I thought it may have been too early to detcect hcg. Ithen had light bleeding on the 26th of april which turned to quite heavybleeding, severe, sudden cramps and clots of pinkish and bright red blood on the27th. My period is usually average, not like this. My cycle is also every5weeks. Why was I bleeding twice? Could I be pregnant? Or was I pregnant? Thanks