I have an extremely bad back and am currently prescribed tramadol and flexeril. I have, in the past, taken Vicodin and flexeril for my back without problems beyond the usual Vicodin wonkiness and more sleepiness than usual from the added flexeril.
What I would recommend is staggering the dosing. IOTW, take your Vicodin, and three hours later, take the Flexeril, though I don't know how much good it will do for cramps of that nature as it isn't a smooth muscle relaxant.
Actually, NSAIDS work very well for menstrual cramping. Ibuprofen or Naproxen might be better bets than Flexeril. If you do decide to take the Flexeril, and are really worried, I'd suggest having a "sober sitter" with you the first time.
The combo won't kill you, but if you are super-sensitive, there is a risk of dizziness and possibly falling.