My personal opinion is that a flare should ultimately be defined by symptoms, but many doctors do tend to use things like ESR/sed rate, c4/c3, DsDNA, protein in urine... to help define a flare for them in addition to symptoms. A drop in c3/c4 is one way doctors will sometime define a flare through the use of blood tests at least.
I know that complement isn't often tested after diagnosis for the simple reason that signs, symptoms and other test results are sufficient to show if the disease is under control or not.
If there are signs of increased disease activity or new disease activity especially something so perfectly obvious as lupus skin to anybody with open eyes why would anybody doubt that something is going on.
I am pretty sure that complement levels don't always reflect worsening disease anyway as that would be in keeping with everything else unpredictable and wayward about this disease. For some inexplicable reason I think it is unlikely that complement would change for skin disease developing or worsening but I might be quite wrong.
Is your doctor saying that you can't be in a flare because complement is normal?
I hope you can get some help soon. I think skin problems starting after diagnosis of SLE must feel like the last straw. Looking bad as well as feeling bad sort of thing.