Hi everyone- could I possibly be pregnant?

Hey guys- I had unprotected with my long term partner last week and was put on antibiotics that following Tuesday. Late yesterday and today, I noticed some spotting. Initially, it was a cherry red, but today, there is a lot more and its more brown or red. I'm only 13 days into my oral contraceptive pack and my period is not due for another two weeks. Is this just breakthrough bleeding or this thing called implantation bleeding? I have been feeling nauseous for a day or two but I think its due to my antibiotic, which doesn't take good and from what I read could cause such a symptom. No morning sickness or swollen has occurred. Any insights?