Knee pain

Last night my knee went out just as my dad needed me to help him walk down the hallway. It was a very pitiful sight, me limping along hanging onto a cane to ease the pain in my knee and trying to help steady dad as he walked. I'm not sure just what happens to my knee, it just starts hurting occasionally - really *hurting*. The bad part is, particularly when I need to help dad walk, that some times when it goes out there is a chance that it is going to completely fail and I'll go down. It has only really gone out once, and that was roughly 15 to 20 years ago, but that was sudden and painful enough that I cringe at the idea of it happening again.

One would assume it was stress in the joint from helping my dad walk, but it has nothing to do with that. Yes, he has squished me down by a full inch, but I had the trouble with my knee long before that. Next best guess would be that it was caused by weight stress, but the trouble was there many years before I got overweight and has not really got worse in proportion to the weight I gained from stress. My best guess is that it is some kind of rheumatoid arthritis or something, but that is based on the knee being worse in cold weather - which I guess could be a symptom of a lot of things really. Huh?