I have allergic type symptoms too since becoming severe/getting covid, and hands going bright red after I eat high histamine foods like curry. I decided to take it seriously after two occasions where I ate a heavily fermented food (kimchi on one occasion and yellow bean paste on another) and had intense symptoms.
After I was put on a low histamine diet a year ago I noticed a lot of symptoms decreased significantly, but the diet is utterly miserable and hard for my carers to stick to. I have tried H1 antihistamines twice, ceterzine made my brain fog worse and a recent trial of fexofenadine coincided with a period of frightening crashes. I have a backlog of famotidine I keep meaning to try.
I really want to reintroduce normal foods but every time I try I crash and feel unwell. This may be correlation not causation though, as it is miserably easy for me to trigger PEM.
I know someone with ME/LC who suffers terribly with MCAS/allergies, and have heard a lot of similar stories in support groups.
I do sometimes think the MCAS clincal category is too broad (Afrin claims it can cause everything under the sun), but imo there is definitely something happening here that sometimes accompanies ME/CFS and worsens with PEM.
Attached two more extreme examples of said swelling and redness in hands.
Edit: Since covid/becoming severe I also find that black pepper, which I used to love, now burns my mouth, and if a crumb of it stays on my tongue it almost has a numbing effect. so i had to stop eating it.