120mg of pred is 2x the normal high dose of pred. Hopefully you will only be on it for a very short time and taper to something lower. I am curious why your doctor would start at such a high dose. Is your doctor a rheumatologist and is he/she experienced treating lupus? Not all are. In addtion you are taking the 2 other drugs. Plaquinel is often the first treatment for milder cases and azathioprine (imuran) is a prednisone sparing, immuno suppressant that is often used as a long term treatment for many AI diseases.
I would ask the doctor before taking any pain meds. These drugs can be tough on your liver and kidneys and adding an nsaid might cause a problem. If you are not getting relief from pred which is a much more powerful antiinflammatory than nsaids I would ask your doctor about something for temporary pain relief. Pred should do the trick but it might take a few days.