What Causes Delay In Periods?

Hi doctor. I m 23yr old, I got my last periods on 20 October I had unprotected sex on 28 october. Till now I didn't get my periods. I have done pregnancy test in December n January but the result vs negative. My Doctr gave me meprate fr 5 days. On sunday my course of meprate vs completed. Till nw I haven't got my periods. M vry much worried. 1 more thing I do have d problm of pcod in past n recently I have gained weight. Kindly suggest me something, m vry much tensed n worried