Sympathies. When I first went on Depo-Provera, which has more or less the same drug as Mirena, I had a few instances of random bad cramps from nowhere (like, not as part of a real cycle). Then the whole system just shut down, yay. Hopefully the same thing will happen for you--a lot of women on Mirena don't get any periods.

Muscle relaxers can help, if you happen to have any lying around--I used to take flexiril and lo, it was good. Those will totally screw your productivity, however.