I'm sure you know that there's not specific test to say you positively have lupus. The C3 and C4 tests can show if they are low that your lupus is active. The fact that your C3 is low if just may be showing that you've got a bit of activity with your lupus. Since you've got tumid lupus the C3 may be related to that. I'd suggest that your doctor keep a close eye on it.
I hope this helps and keep us posted.
dx fibro, SLE, glaucoma, cateracts, bells palsy, depression, migraine headaches, gastreopaersis, chronic anemia, MDS (Blood Cancer). Mediport and tons of meds.