What Causes Cramping And Bleeding After Insertion Of Mirena ?

8 months agod i had my daughter then 2 months later I got the Mirena put in. I was feeling fine, with very little cramping and bleeding. In february I started training for a marathon and May 1st I participated in my first walking 13.1 marathon. Ever since then when ever I exercise, like walking, swimming or running, I have been cramping up and bleeding very heavy. It happens every day after I exercise and then goes away the next day. I had my husband check and he can still feel the string so I know it has not fallen out. I called my gyno and she said spotting is normal, but i find it weird that it happens everytime after I exercise and the bleeding and cramping is becoming more and more stronger after each time I exercise. What do you think it is? What should I do?