I would suggest a new Rheumotologist! Don't make her suffer, find another doctor. I'm a strong believer in Cleveland Clinic, I dont'know where you are, but it's worth the trips.
I meant to add that anything that stimulates the immune system, like a common infection, can trigger her immune system into overdrive. This can be why she is experiencing so much joint pain when she gets an infection - the body attacks not only the infection itself, but healthy tissue as well.
If meloxicam is all she's taking, I would find a different rheumatologist who will treat her disease more aggressively. There are a wide variety of medications out there now that can greatly slow the disease process and give her a better quality of life.
I have RA, not lupus, but they are in the same autoimmune "family" and often the same medications are used. Since lupus can affect the internal organs, I would take her to see another rheumie until you find one that will try something other than or in addition to an NSAID like meloxicam. (I also take meloxicam, but in conjunction with three other meds for RA that go after the disease process itself, not just treating the symptoms like meloxicam alone).
Blood work is only one of many diagnostic tools, not the yes-or-no definitive answer. Autoimmune diseases like lupus can be extremely hard to pin down because they can manifest in wildly different ways from person to person. Good luck!