Please help! This has been stressing me out!

I had my LMP on June 27th, and expected my next on July 25th. My periods have always been consistently heavy, needing a pad change every 2-4hrs (sorry). I also always get very painful cramps. I had unprotected sex on July 12th. The day my period was due I experienced light pink bleeding, only noticeable when wiping. The two days proceeding have turned into a heavier flow, red/brown. The entire time I've had this bleed, i havent had cramps! This is the first time I havent had cramps. Does anyone know why? Is this my period or implantation bleeding? Can it last more than 3 days?? (On day 3 now). Keep debating if i should see a doctor. Please offer any advice you can. Thank you!