People with fibromyalgia can have this problem as well, I have had it myself and I have fibromyalgia. It can also be anemia.. do you have any warning? Or do you just go down? Any lightheadedness? And vertigo-like sensations?Dizziness?

Or do you think maybe your limbs are giving out without warning? Like my ankles do sometimes.. one minute I am walking across the parking lot and the next I am laying in the parking lot because my ankle went on me.. or my knee buckled suddenly, or my hip locked up....and sometime si don;t know what caused me to fall.

I think you definately need to make a visit to the doctor to be sure of what's going on. Better safe than sorry... Let us know what you learn. and Good Luck!Faith