Can a woman’s body become immune to her birth control?

Well your liver breaks down hormones and when it's function is increased then it is possible that your body has broken down the progesterone and estrogen to the point where you are ovulating. If you have had a history of pregnancies on the pill then this is not an effective form of birth control for you. Also just because your HPT are negative doesn't mean you aren't pregnant. It could be that your body isn't producing enough HCG.