What Causes Pain And Cracks In Knee?

Hi, Im a 20 year old girl. I have had knee problems since I was 9years old and it has progressed quickly along the years. Everytime I bend my knee, it cracks with major pain. I can barely move my knees anymore. I can't bend down, I'm scared that they will give way. I feel like I have 80year old knees and Im 20. Its bruised on the outside. Ive never injured my knees durring childhood games or sports. My knees have given out and made me collapse 1 time, well I was at work. Im scared it may happen at anytime. Everyday it gets worse and worse. Can you help me in anyway with what this might be please? Thank you!