Brown Spotting After Sex

I am on birth control however I have been taking Amoxicillin for ten days. While on it, around day five I began very light spotting a brownish color that continued for a couple days and stopped. I finished my last pill on Friday and had protected sex that night as well as Saturday night. Sunday evening I had more brown spotting, very light and dark with short cramping. Is this just because of the antibiotics affecting my hormones or could I be pregnant? A condom was used both times and no slip ups occurred to my knowledge, but mistakes happen. My period is very regular and I have never experienced spotting inbetween periods so I am concerned. I can't get in to see my gyno anytime this week. Should I purchase Plan B, I'm still in the five day window, or is this just a reaction to the Amoxicillin?