The condition is called proteinuria and should be treated. This condition is common with lupus nephritis. Your kidneys normally do not filter out the larger protein molecules but when they are attacked and damaged by your immune system sometimes they do. The immediate symptom is edema in your feet, ankles and legs. Your blood does not retain fluids as well and it seeps out into the surrounding tissue.
I had that condition and twice gained 15 lbs of water weight in just a few days. My case was a class II one and there appeared to be little permanent damage to my kidneys.
If not treated you can damage your kidneys further and it is potentially a serious condition. Normal treatment is with ace inhibitors like lisinopril and meds should do the job fairly quickly.
http://www.emedicine.com/MED/topic1597.htm
Bill