you've asked this question before mamas_girl and i'm still not sure i understand what you mean?
if you are fasting and you are losing weight then this should continue irrespective of whether you are menstruating or not.
i am fasting at the moment and just got my period. my only symptoms were that i felt really depressed for a few days prior.
the only way you will know for sure is to actually try the fast and see how you go!