Hi Doc 6 Months Ago I Take Acl Surgery......then I

hi doc 6 months ago i take acl surgery......then i take physio therapy....now i am good to walk......but when i sit in a chair fer a long time like 2 hour after that when i try ti stand up then i feel pain and i dont straight my leg....but after few time i can walk normally blike previews time....why is this happened....