What Causes Delayed Periods With Negative UPT?

hi doc, im 27 years old, i have an irregular menstruation, i and my boyfriend had sex with the used of withdrawal method, i got delayed for 1 month so i took a pregnancy test for about 3 times already 1 test every week but it always negative, now its been 2 months but still i don;t have yet my menstruation, is it possible doc that i am pregnant? im worried pls help.