Same for my wife. Actually, just before it starts. When she feels it coming on it is my "husbandly duty" to make sure she has a good orgasm - and then (she says), "no cramps".
Sex during her time is, well, messy. That's what old bath towels, soap, and water are for. You'd be surprised how many positions are possible in the shower (without breaking your neck!). As for oral - no thanks.