Without more info then it's hard to comment and so to keep it general
lets say there are three tests commonly employed to assess complement.
The total, C3 and C4 and so it is hard to know which is decreased in
your case.
In general it indicates immune activation and some of the most common
ones are rheumatologic causes like lupus or glomerulonephritis. A
sedimentation rate is helpful in screening highly active collagen
disease. Joint and flank pain would wonder about the kidney and a
urinalysis would be helpful as again immune complex disease can impact
the kidneys.
Depending on history and physical and the type of info from the labs
like is there a virus lurking ie Mono or hepatitis. The joints are like
filters and any immune complex can be trapped in there with resulting
joint inflammation as it activates complement. Sometimes one can have a
normal or even a high blood complement level and have a low joint fluid
level because of local complement activation.
Without other symptoms then dermographism probably not related.
Urticaria can be associated with viral infections and some viruses can
cause immune complex depositions and arteritis. Liver enzymes would be
interesting.
The results mentioned in your post are not specific for anything by
themselves and again further testing or evaluation is is needed.
I would hope that the sedimentation rate is normal with a CBC that is
also normal without atypical lymphocytosis and normal ALT and a normal
urinalysis and a normal CRP as an indicator of inflammation. It is
interesting to note that people with lupus will have low complement
levels but a normal CRP.  There is minimal activation of cytokines and
so CRP can be used to rule out infections especially bacterial
infections that can also activate complement.
Sorry to ramble on but those are just ideas off the top of my head and
I am not a doctor for what it's worth.