I took an at home pregnancy test yesterday and it showed a light positive line, which I have always understood to mean the test is positive. My doctor has me taking my birth control in a way that I only have one menstrual cycle a year in order to control pain associated with possible endometriosis. I take a pregnancy test from time to time just to check since both of my children were conceived while I was on birth control. This morning I experienced some bleeding. I read online that implantation only causes spotting or extremely light bleeding. Mine was not heavy, but also not really light. I also read that bleeding can be caused early in pregnancy from having intercourse, which my husband and I did have last night. I just wanted to know if I could still possibly be pregnant, could have had a false positive or some other explanation. Also, if I am pregnant, how would I determine a due date when I only menstruate once a year?

Of course you could possibly pregnant. As you know, birth control only reduces the risk of pregnancy, it does not eliminate it. Re-test with an HPT and first morning urine, and if you still get a positive, or have concerns about the negative, see a doctor.