I have a few quirky, hopefully cheaper ways to weed out whats wrong. If its growing pains, the pain is in the large bones of the legs, not the joints. It is significant for 3-5 yr olds, and is common towards the end of the day. Try increasing her fluid intake, massage, warm packs or a warm bath, and tylenol/ibuprofen.
If you think its restless leg syndrome, try giving her some tonic water due to the quinine in it. The doctors used to prescribe quinine by itself.
If you think it is charlie horses, try putting a bar of soap under her sheets. Any kind will work except dove or ivory. Quirky but it works.
If the pain continues, lasts all day/night despite all your efforts, get a second opinion.
Good luck! Both my 4 and 3 yr old's have growing pains and they wake up at all hours of the night crying, sometimes having a hard time walking or bearing weight on it. I hope you find an answer that works for you.