What does it mean when I feel and hear cracking in my knees and elbows and it is also painful?

I often get cracking in my knees and elbows on a daily basis. It first started with my knee after a sprain, then with my other knee that was never sprained. It then began with both of my elbows, usually one more than the other. When the cracking happens, it causes pain that can last from a few minutes to weeks. I have had physical therapy for the sprained knee and it began to work. I am concerned because it is now happening in both knees and elbows. Any ideas what it could be?