Systemic Rash Systemic Lupus Erythematosus Or Rheumatoid Arthritis?

Systemic Lupus Erythematosus or Rheumatoid Arthritis? - systemic rash

My symptoms are extremely painful to walk, rash, blood in the urine, anemia (iron infusions needed), headache, extreme fatigue, etc.. I saw a doctor has X-rays, blood tests and physical examination. Recently I called and told me that my lab results came back and told me to either lupus erythematosus or rheumatoid arthritis. She said that my blood tests showed many anomalies, and although infection. She wanted that I immediately started on steroids. She put me on (tablets of 10 mg of prednisone) for 10 days until my follow-up with a rheumatologist. Now my question: What is lupus and rheumatoid arthritis?
Are they imitating 2 Disease? It reminds me in 5 days with the results of other tests. Has anyone experienced personally? She said that my disease is treatable, but not cureable. I am still very young and it's a shock to me and I do not know trouble dealing with what I try, you can guess my new doctor to answer questions later.