Chemical pregnancy while on birth control?

I am currently TTC. However last July while on BC something strange happened. I went for my Annual at PPH and of course they did routine questions and uring preg test. OUT of the Blue they tell me I'm pregnant!!!! I refused to believe it so they took another test-positive again. They even gave me a due date. I went home unconvinced. Next day I insisted on blood test-and it was negative. They never explained anything to me. Could this have been a chemical pregnancy????