12 year old female with significant pain in one leg--appears worse with exercise and pain moves from knee upward to top of thigh and sometimes from knww to ankle. At times, pain involves entire leg. Advil or tylenol helps. Child has to walk on toes when pain exists. pain has left for days and suddenly returns. When on Advil, etc. pain leaves completly for a coupl eof hours, but returns. Has been happening for about 4-6 weeks. Any suggestions as to causes?