What Causes Vaginal Bleeding After Riding Bike?

I have had Mirana for about 3 years. In that time I stopped having a menstrual cycle after the first year. About 6 months ago I got a bike to ride for exercise. Every time i ride it have internal vaginal bleeding. It doesnt matter when I ride it could be days apart or months apart but I always bleed.The seat is not hurting me and there is no damage to my vaginal area. The bleeding normally occurs hours later or the next day.