I know some sources of Quercetin are Citrus derived so you may want to look at limiting citrus fruits from your diet if you haven't already since they tend to be allergenic anyway. Also does your Quercetin contain any Bromelain? Could be an allergy to Pineapple or a mold that it can harbor.
You probably have already but just to be sure Wheat, Dairy, Soy tend to be at the top of the list of irritants you would want to minimize & most people who have sensitivities such as myself do MUCH better off of them. Keep the Sugar in your diet low to non-existent as well if you haven't already.
Since most histamine supposedly develops in the intestinal tract it never hurts for any allergy sufferer to take daily fiber shakes to ensure regularity. You can always add in some caprylic acid to the shake if you suspect some candida overgrowth but you don't want to use it for too long as it can stress the liver.
In addition try using Triphala to increase the BMs so at least when you do react you can flush it out of your system sooner. It can be done with Vitamin C in a single dose of about 6+ grams in emergencies when you need to flush the digestive tract asap to clear out a bad reaction but I wouldn't recommend doing it too often. This is what I've done in the past and it worked well when I had a very negative allergic reaction.
Theres lots of Adaptogenic herbs that Hveragerthi recommends which you can try. I'm quite sensitive to most things and I found Astragalus & Ashwagandha both are ok for me & should be of benefit to you. Amla though is nice as well, doesn't seem to give me any problems if not a little gritty : ). Theres some in Triphala with a couple other herbs if you wanted to use it that way.
I wouldn't worry too much about the Quercetin reaction, there are various allergens that could have easily been contained in it. Especially if it also had Bromelain.