Could I be??

So today my period is five days late and I've taken 2 pregnancy tests that came back negative. I'm not on birth control and we use withdrawal as our primary form of contraceptive. I was on an antibiotic in the second week of March could that have thrown my cycle off? I'm never this late. I also had a tiny tiny bit of bleeding the day I was supposed to get my period. Could it have been implantation bleeding? Any advice and experience with this will be much appreciated girls!!