I Was On Sulfamethoxale For About A Week And I

I was on sulfamethoxale for about a week and I was supposed to receive my period.. I've been on birth control pills for 2 years and take them religiously. I've also taken a pregnancy test and it was negative. My period is now about 2 weeks late. I was also on 1500 milligrams of other antibiotics?what is causing this?