Sharp pain in back of thigh radiating down leg. now swollen and can't put any weight on it. have back problems and fibromyalgia. what could this be?

Few problems : Sharp pain radiating along the back of the thigh may be caused by a "pinched nerve" involving the sciatic nerve or lumbosacral nerve roots. This may be directly related to your back problem. Swelling & not having difficulty putting weight on your leg may be related to the same or a different condition. See a doctor asap to sort these out so you may get the proper treatment.