I've heard of this being very common amongst pregnant women (it certainly was for me....uggh ick eww!).
Otherwise, as Lupus is a disease that can cause inflammation throughout the body, you may have inflammation of the of mucous membranes of the mouth (stomatitis). Or it could be a side effect of certain medications. Do you take anything such as clozapine, isoproterenol, pilocarpine and reserpine?
Excessive saliva is usually a temporary problem and rarely a cause for concern. However, if it continues or has been this way for some time, it is best that you contact you doctor to find out exactly what the underlying cause could be.
Best of Luck
Peace and Blessings
Look For The Good and Praise It!