Running out of luck... please help!!

This is how my chart has been this month. I had lh surge on the 13th and was expecting to get pregnant but unfortunately i started bleeding yesterday but only lasted during one trip to the bathroom and then this morning, same thing. Could this be breakthrough bleeding or implantation bleeding? Ive been pregnant twice before and this has never happened to me. We are desperately trying for a baby and i havent been on the depo since april... is there any slight chance i could still be pregnant??? TIA!!!