Hi there...pain in the posterior aspect of thighs as you are describing could be due to number of reasons like sciatica, muscle strain, meralgia parasthetica, any nerve injuries. Also sensations and movements in those areas will also have to checked. So kindly consult a neurologist or an orthopedic for further evaluation. Till then it is best to take rest and do not over strain the muscles. You can take a painkiller if the pain is severe and you don't have any gastritis or drug allergies