What Causes Irregularities In Menstruation?

hi my name is julia

iv'e been taking birthcontrol for 3 weeks now and the reason i started taking it was because i have been getting my period irregularly and my dr. asked me to take them because in my future i wouldn't be able to hav kids and she said birthcontrol will give me my period monthly well last weekend i had sex and my bf didnt use a condom i don't know how the birthcontrol works and i need help.