What Causes Spotting And Vaginal Bleeding During Exercise?

Im 38 5ft 8 195 i have recently over the last 6 months lost 45 pounds through diet and exercise. I have started to pick up the pace in exercise for i have seemed to reach a plateau in my weight loss, since then i have beee bleeding like old blood not a heavy flow some spotting and i can see tissue floating in my urine this has been going on for about 5 days ive had spotting with exercise but not like this.