My daughters leg shape is unusual and is causing her pain.?

My 13 yr old daughter has very thick legs from thigh to ankle and consequently her ankles are very thick. She tends to walk with her heels lopsided (inwards). After some physical exercise at school a few weeks ago she has hurt her knee, it is very painful at the back of her knee. She also seems to be flat footed. I am worried about her, she is in a lot of pain and can hardly put weight on her left leg due to pain in the knee. She wears very flat pumps to school which I feel doesnt help at all, but its all the fashion. She is tall for her age and is not particularly overweight. Am taking her to the doctor tomorrow, any help would be appreciated, thank you. I so feel for her. She will not wear skirts and it is knocking her confidence terribly.