My 89 Year Old Mother Has Been Experiencing Excruciating Pain

My 89 year old mother has been experiencing excruciating pain in the front inside of her upper thighs. She says it feels like it involves the nerves, is not muscular pain. It feels like the nerves are contracting. The pain is excruciating and brings her to tears (my mother never cries). It occurs only occasionally. She doesn't know what brings it on. It frequently happens at night when she's in bed, though not always. Her doctor was no help. He prescribed muscle relaxants but said he didn't think they would help and they didn't. Any idea of what could be causing this?