Birth control and bloating

Is it possible to get pregnant on a low dose birth control? I'm 25 and My boyfriend and I have been having unprotected intercourse and I have been having weird spasms and cramping almost like a UTI but it doesn't burn when I pee. And I have had some major bloating and cramping to the point it feels like someone is twisting my intestines and period isn't due for 2 week. What are the odds? Don't know if it possible but I have taken 2 pregnancy tests and they both came back negative. But my mom was telling me she has similar symptoms when her birth control failed and she found out she was pregnant with me, she went to the doctor thinking she has a UTI and they did a blood and irons sample and told her she was pregnant... Any thoughts?