Very interesting. I'm about a week away from my period right now and developed this pain in the right side of my back yesterday. Thus far, Motrin, Naproxen and transcendetal meditation have not worked. I wanted to work out and now all I can do is sit like a slug in a chair because I'm in pain.
Anyway, I noticed years ago that I become much weaker the final week before my period starts. I have to use lower poundages in dumbells and stick to low-impact cardio until the day after my period starts. Then, everything goes back to normal.
D.