Assuming your cramps are for the usual reason, that might have something to do with it. I noticed when I was taking karate that I was noticeably more sensitive to any kind of pain or discomfort, including plain old sore muscles, just before and during my period.

And, of course, the older you get the more quickly your muscles lose their conditioning and the quicker they are to complain about it. I'm sure you'll readjust to your old routine in a week or two, provided you dont get carried away and hurt yourself. Well, maybe a week or three.