How long is this injury? I guess u hv hamstring injury. There must be a minor tear or strain in hamstring muscle. If it is less than a week, do rest, icing, den dont overstretch it, just do ankle movement in long sitting both with knee straight and bend. U can start stretching after 2weeks but it depends upon sverity of injury, U can do deep tissue massage after 4week along with stretching and strengtheing exercise 4r hamstring muscle.. I guess in urs caee its more than 4weeks, pain is nw due to adhesion of scar after healing in muscle fibers so u have to do massageEyo loosen those scar tissue den stretch muscle.. done overstretch it.. do it in pain bearable aintensity.. its natural to feel intense pulling stretcing pain but u hv to do it in urs  pain threshold...  dont forget to do strengthening exercise 4r leg muscles.. if u want detail of exercise u can write me at https://www.facebook.com/PhysiotherapyKnowledgeWonders/ https://www.facebook.com/3Dmovementpattern/