I had a bad right hammy that gives me trouble every once in awhile during runs, like at the end stages of a marathon. I can deal with that. But recently what has been happening is that it will start cramping up after a run. Like a few hours after a regular 6 or 7 mile easy run I'll be sitting in a restaurant and it will seize up on me. I also often can get it to cramp if I do back bridges or hamstring exercises. Any ideas as to what causes something like this?