How does all this go about

Can someone help me understand this more . I had broken my virginity at the age of 17 , but I was on birth control . Never got nutted in w/ the guy.. At the age of 18 I stop taking birth control but right when I made 18 , the sex became more  intense & well u get the point . 😩 everyday having organisms in me . But I didn't get pregnant until around 5 months later . I honestly wasn't trying but still .. I don't understand how it didn't been happen?