I have been on birth control pills for 3 years and I always take them regularly. I was put on amoxicillin last month and was told it could interfere with my pill, so my boyfriend and I used condoms the whole month (I continued taking my birth control as well). I finished the antibiotics on the wednesday before I stopped my "active pills". I had sex without a condom on the night I took my last "active pill". I was worried about the fact that the antibiotics may still interfere so I took the plan b the next morning, on the first day of my sugar pills. Normally I get my period on wednesday or thursday of the sugar pills and it's now friday and still no period. I took a hpt last night and it was negative. Could the Plan B have messed up my period?