First off....gentle hugs to you for all you're going through. I'm sorry you're having pain and feeling miserable.
I'm not sure if I have answers for you...but I'll try. I've had Lupus for 27 years, and have come to know what Lupus likes to do. It likes to move all around to different joints, on different days, and cause pain and inflammation in those joints. One day you'll have it in your fingers and wrists and elbows and knees and then the next day, it might be only in your shoulders and ankles. It is so very very frustrating trying to keep up with it. My worst joint pains were always when I was in a flareup, but sometimes I'd even have them when I wasn't really flaring. I'd just wake up otherwise feeling okay and then within a few hours one of my joints would be in agony. It doesn't seem to have any pattern or make any sense. Sometimes I might have it in a joint that I had been overusing the day before, like my hands digging in the flower garden, but most of the time the joint pains just came on randomly. And sometimes, like the pain in your knee joint, it would last for days or weeks and get so bad I could barely walk. Those are the times that this illness gets unbearable....when you feel like you can't take another day of this. When I had the worst joint pains, especially in my knees and hips and ankles (the weight bearing joints) my doctor would always give me pain medications that was stronger than over the counter kinds, but I would use it very carefully, only taking it for as long as the joint pains were their worst. Have you talked to your doctor about pain medication for when it gets unbearable? Also, I believe the reason your doctor wants to get xrays of your knee, is to make sure this pain is not caused by an injury to your knee. He probably wants to see what's going on inside the knee joint, like inflammation from arthritis.
The cracking and pain in your jaw might possibly be from arthritis too. Over the years with Lupus, I've had several times that I had pretty bad arthritis in one of my jaws or both. And when it was inflamed from the arthritis, it would always crack. I've had times when it's been pretty painful to open my mouth to eat and I had to sip liquid type foods until it eased up. Can you call your doctor's office tomorrow and ask to speak to the nurse and explain to her that you didn't understand a word that doctor told you, and ask them to look it up in your chart and explain it to you? If you don't understand something the doctor explains to you, even just a word, you have every right to ask someone to help you understand it. It's your health and your body and you have a right to know so you can take care of yourself. If your jaw cracking is extremely painful and you are unable to open your mouth to eat because of it, you should call your doctor right away and ask them what to do.
Maria, I'm not sure if I answered your questions or helped you, but I wanted to let you know I care that you hurt and you're confused, and I didn't want to just let you sit and wonder. It sounds like possibly what you are experiencing is from the Lupus attacking your joints, but this is only my opinion of course.
Lastly, if there are any words or anything you don't understand when I reply to your posts, don't be afraid to ask me to explain what I've written. I very much understand how hard it is to understand English for someone who might be still learning it. I'm always happy to help you understand.
I'm really sorry you hurt so much.....