My girlfrd is 43years old she has been having sharpe pains in the back of her thighs lasting days at a time. They get so bad she can barely walk, what could be the cause.

hi, Anon.

Thank you for posting.

Muscle pain, spasms, and cramping in the back of legs can develop for a variety of reasons. Simply straining the muscles can cause discomfort between the thighs and calves. In some cases, medical conditions may lead to muscle aches or sharp pain in the back of upper legs and behind the knees.
The pain might be caused by sciatica, (pain radiating along the sciatic nerve, which runs down one or both legs from the lower back.)

She needs to see a doctor find out what is going on. There is no way we can know with any certainty.

Helena