first of all, see your doctor and get the proper tests completed.
you can always do the elliptical machine, or the stationary bike, or even the rowing machine...all low impact exercises with good cardio benefits. you may want to try walking instead of running until you figure out what is causing your pain.
you could also do some strength training exercises...basic exercises-light weights but increased reps. (i.e., lunges, squats, leg extensions).
but absolutely see a doc first.