Missed Period, Negative Test, Antibiotic effects?

I have been taking birth control for 6 months and have never missed a period or a pill, but i have always had a little spotting between periods. Last month I got strep and was taking penicilin for ten days, I had major breakthrough bleeding and no period once on my inactive pills. I am sexually active but I have used protection everytime but once in the last two months. Two days after the missed period i took a hpt and came back negative...Could i be pregnant? Could the Penicilin have effected my period? Is this normal?