I don't thik that is necessaarily what people would believe. You could always simply point out why - which I think a far easier thing to disclose than the fact you take it. Incidentally, I think it quite common that young women be on birth control to regulate their cycles. Lastly, I think slipping up and mentioning you are on it may be far easier than dealing with your menstrual issues. Give it some time and you may come to agree.