H.M.
I've been reading some of the responses, because I feel much the same way. I just had baby number 2 (a beautiful boy) and it has been 6 months already. I also have fibromyalgia. It is possible this is what is going on, or even just a deterioration of muscle from both bedrest and estrogen. When you are pregnant, estrogen levels are much higher. Estrogen actaully causes you to loose muscle mass. This can continue after pregnancy if you are nursing.
First and foremost, consult your doctor. Very important! Second, depending on what he/she says, eat a well balanced, higher protein diet. Third, exercise. BUT, take it easy. Your muscles have a lot of building to do. Go on relaxing walks with the baby in the stroller. When ready, lift very light weights, even use canned vegetables. Do 5-10 reps. Something very simple is all it takes to build muscle. But of course, do this all on your doctors approval.
Best of luck. Know we are all here for you!