Knee problems while bending and standing?

Lately within the past couple weeks Ive been dealing with sever knee pain. I can't bend it while standing for long periods of time. And while bent I cant straighten it out, both instances involve sever pain. I tore my acl/mcl in other knee 5 years ago, and it feels like the pain you get from rehabing that injury. Any ideas of what it can be?