I Am 17. I Had My Last Period Was On

I am 17. I had my last period was on 10th November,2015. I had oral sex on 28th November,2015. I went to a dermat for my bacne problems and he prescribed rablet 20, azinok 250 and a vitamin tablet. I continued following this until 10days and then stopped. I still haven't got my period. It's 5th jan 2016. I have no symptom of pregnancy. Nothing at all but i have pain just under my sternum. what is wrong with me?