Steroids and other immunosuppression drugs do lower your immunity but everyone is quite different. Some patients have many illnesses, minor or worse, others, like myself, don't get sick. I have not had a single cold but had a single sinus infection since getting sick 7 years ago. about the same as before getting lupus and polymyositis....rarely sick.
What are you taking and what it the dose? The greatest risk is at high doses (like prednisone at 40-60mg daily or even higher). Lower doses do not totally disable your immune system.
Follow common sense if your condition begins to drop lower in the bronchii or lungs and see a doctor. Have you looked into a lupus support group in your area? It would help you cope with your disease. It sounds like you have a mild case and while you should be cautious, don't worry so much.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 25mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.