What Causes Delayed Menstrual Periods?

i had sex with my partner about two months ago while i had my periods. my cycle is of 15 days. i had to have primolut to delay my cycle around 10 days after i had sex. i got my periods 1 day after i stopped my tablets. i have not got my periods for the last 1 and half months after that. i just needed to know if everything is normal and if one could get pregnant by having sex during menstruation. please note that my regular cycle is of 15 days (1st,15th,1st,15th...)