Skipped period. When i will get periods.

My periods comes atleast 2 days late every month. Last period was on 24-28th Jan,2017.Had unprotected sex on 4th February.Partner ejaculated sperms after taking his penis out. Unfortunately, I didn't get periods im the month of February.I had exams in the first week of March.I consumed papaya juice on 11th and 12th March. No effect.Took pregnancy test on 13th morning (Monday).The result was 'one line' (not pregnant.) For my complaints of period got skipped, my friend suggested a tablet called 'meprate'.I'm taking it from the next day (14th - Tuesday) twice (after breakfast and dinner) per day.Today will be my 5th day to finish off all 10 tablets. Also, once again I checked if I'm pregnant on 17th March (Friday). It was again one line (not pregnant). I would like to know the reason behind this and if really I'm not pregnant. I guessed stress might be the reason behind this as the past three months were kind of stressful. But still I would like to when I will get periods.