1. SEE A DOCTOR, NOW!
2. Could be any of a number of things going on, most of which I (not having the expertise) won't go into; however, your end of the approach could be at fault (i.e., do you plant and heave? or slide?)
3. For short term help there are a number of braces you might look at: I use (at times) a blue neoprene brace with neoprene support ribs on it. Helps a lot. I'd also suggest glucosamine chondroitin (if that's the exact name I'd be surprised, but it's very like that). When I remember to take it daily it helps (over time).
4. SEE A DOCTOR, NOW!
--------------------
"Things fall apart; the center cannot hold; mere anarchy is loosed upon the world,"
I'm Ragnar Floggurass, and I approve this message