Jenny,
It is not being overly cautious to investigate edema and low bp. Kidney problems are very common with lupus and often can be easily treated eliminating the danger of losing a kidney or worse. Proteinuria is the most common condition and if treated early on you should be able to avoid permanent damage. The most common treatment is an ace inhibitor like lisinopril and lasix or some other diuretic. Since you have low bp a specialist might opt for another treatment.
At the very least he will run a battery of tests and possibly a kidney biopsy to see what is causing your problems.
I had these problems and had naturally low bp which later shot up after 10 months on prednisone. Still take an ace inhibitor and a low dose of a diurectic and my kidneys are doing fine....no swelling in almost 5 years.
Bill