J., even though you are on birth control, sometimes your egg came become fertilized and still try to implant as it would for a normal, non contraception conception, and your body will begin to respond to the fertilized eff by starting to go about as if you are pregnant, because that is basically what thebeginning of pergnancy is- a fertilized egg. However, when on birth control the egg cannot implant, the body flushes or aborts the fertilized egg and begins the whole process of your normal routine once again...kind of like a reset button
This may not be of comfort to you to hear it, but it is quite possible that this is what happened to you.