How To Heal Ailing Knee?

i have severe pain in back of my left knee after i stand up and it becomes difficult for me to straighten the leg although there is no locking but i cant straight it due to pain but after sometime pain reduces and i resumes to my duties. i feel like my knee will jump out to the left side. i am facing this problem from last months but yet i have not consulted an doctor . i cant stand for long period please help me out