Implantation bleeding or period

Patient: My last period started 9 June 2015, meaning I was to start 2 July 2015 again, However in the time leading up to my period I was experiencing extremely sensitive nipples and yes I am sexually active ( Married ). My period did not start and on the 5th day of it being late I had the really light pink blood that came out and for 2 days was was heavy, clots and dark red and then as fast as it started it ended. I 3 HPT in the time before the bleeding started and they were neg, I am just wondering if this could be implantation bleeding or if it is a normal period. I have not had a late period for 7 years now,Thank you in advance for all you time and assistance

Symptoms: Dark red heaving bleeding