If you go back a few days you'll see one of my posts--I was in the same position you are. I have always had hideous cramps (even while on the pill) and I was worried cuz I usually had to take 3-4 Advil at a time during my period.
Well, I was expecting the worst cuz I went off the pill last month. I cramped a little but I had the best period I've had in I don't know when. I can only assume it was better BECAUSE of the cleanse. I was already on day 9, though. The upshot is I didn't need to take ANY Advil.
Hopefully, this will be your experience. However, I think if you wind up having to take the Advil, it might set you back a bit but if it helps you get thru it and you can continue with the fast, that's the most important part. (We all have to make "adjustments" here and there, you'll see if you read the forum for any length of time.) The long run is the thing with this fast.
Good luck to you.