On cephalexin because of infection. will this lessen the effectiveness of my birth control? had unprotected sex. should i be worried?

Should be ok: The current thinking is that except for rifampin, other commonly-used antibiotics do not alter the effectiveness of birth control pills. This is also presuming a person is not sick with vomiting or diarrhea, or some other condition that decreases the absorption of a swallowed birth control pill. One can look up: http://www.ncbi.nlm.nih.gov/pubmed/12063491