What Causes Scanty Bleeding Or Brown Vaginal Discharge?

Hi, I am 33, about 5ft 11inches and 12 and half stone. I had one incident of unprotected sex with my partner last month after we thought i was still on the last stage of my period but im not sure if i was actually finished or not. Mr last period began beginning of June and normally lasts around 5 days, so i expected my next period to be beginning of July however i started bleeding last weekend and have continued now for 5/6 days initial browner blood and now bright red sometimes and its not a constant flow sometimes less and i think ive finished then heavy again. Should i be concerned?