What's wrong with me ? Why do I keep falling ?

Ok, about a week ago ... I went to the Emergency room cuz my knee was swollen, painful, and had a high fever just in the knee area. The doctor of the Emergency room diagnosed it as Cellulitis. The swelling has gone down. I suspected that a fall a couple days prior caused it to swell. Well, my knee is still abnormal looking. I figured it's not Cellulitis but Bursitis. Just for your information ... I'm disabled *Not paralyzed* but the muscles and tendons from my knee down I guess didn't develop properly growing up. Bcuz of this I walk around my house on my knees bcuz it's difficult for me to walk. My reason I am here is bcuz lately I've been falling down alot. It's not cuz I trip on things ... my legs just give out and I fall. I could figure it could be cuz I'm a little obese that my legs just cant handle the weight and collapse. But could be anything else like Osteoporosis or weakening of the muscles and tendons ? It's abnormal to fall often like this. I'm worried. Any answers will help