Face Forward and Fall

I fell again today. Got up about 6:30 and put dogs out. Tuffee didn't want to come in which is usually. But I don't think she should be out when it is only 25 degrees because she is used to being in the house at night. But she has always been bad about not coming in when I want her to, only when she wants to. I stepped out onto the concrete slab that is our front porch and when I did my knee did it's thing, what ever that might be. Doctors can't tell me what causes my knee to feel like it is trying to dislocate but doens't quite go all the way to be a full dislocation. I call it locking up as that is what it does. My knee joint slips just enough that I can't unbend my knee so down I go. I have been doing this for years but no one other than my hubby has ever seen be do it so most doctors don't even beleive me. I did have a full dislocation about 25 years ago and then again about 15 years ago. Now it trys to do the locking up several times a week. Most of the time it happens when I am sitting down or even laying down but sometimes when I am walking and then I fall. And it is usally faceforward. This time I scrapped and brused my knees, palms of my hands and my chin. Is anyone else having this problem? I would sure like to know.