I have experienced the same thing, even very recently. I was just offering to help a friend of mine who has similar health problems because I was feeling so great. So I offered to help her clean her house. Then with in a day I started feeling the fibro kick my butt again!!!
I put myself back on a couple of supplements I was weaning myself off of and also tried to make sure I was getting rest and was doing daily relaxation.
I guess I just have to accept that this is how it's going to be. Enjoy the good days while they are here, and don't beat ourselves up over the bad days. Try to enjoy the little things in life and pamper ourselves a bit during those tough times.
Hang in there everyone,
Susan