With No Indication The Night Before, I Woke Up With

With no indication the night before, I woke up with pain and weakness behind my knee when I put weight on it. It goes away after I have been walking for a minute or so but re-occurs after I sit down for even a few minutes. I'm 65 yrs old and in good condition and have never had knee problems before this sudden issue. It started yesterday morning and has continued today.