I just finished day 10 of the Master-Cleanse and I had my period while on the cleanse. It started day 2 of my cleanse and I was fine - no probs. I had killer back pains, but I think that was due to detox rather than cramps. Also, bonus...my usual pms symptoms were diminished or gone.
Regarding the nausea...I don't know about the possible benefit of a liver cleanse maybe first? Not experienced enough to say, but may be worth looking into. Good luck. Hang in there - it is so worth it.