I too, have a heavy flow and very bad menstrual pain. I just started the C25K program two weeks ago, got injured, need to rest my knee for 2 weeks and get back on it!
I was under the impression that working out will help reduce menstrual pain and perhaps even lighten periods (which is a reason why I just started this).
When I do get my period, I was planning on taking some sort of pain medication before running. I would also take a breather if I felt light headed and stay close to home. Haven't had to deal with it yet..but I hope this plan works!
Wish you the best of luck!!!