What Causes Irregular Periods With Prolonged Bleeding?

hi! i have irregular periods of my menstruation cycle and after 3months i had my first day of my menstrual period is the 11day of the month and i had unprotected sex on the 19th 21st and 22nd, out of curiosity i did a home pregnancy test on the 2nd of the following month and i turn out to be positive when i went to my ob doctor and had an transvaginal ultrasound and it turned out negative so my doctor gave me primolut n 10mg tab 2times a day for 7days.. my question is will this medicine be effective to induce uterine bleeding? what if im really pregnant? will it induce abortion at early stage of pregnancy? please i need an medical advice on this one thank you!