Only get pregnant while on birth control?

Me and my fiancé are trying for our 3rd child... My first was conceived while I was on the pill, my second child was conceived while on the Nuva ring and over a year ago we conceived while I was on depo... Sadly I miscarried. I've been off birth control for 15 months and have done everything to conceive but Aunt Flo is still visiting regularly. Anyone else ever heard of this happening? Or know what could cause this?