I reckon it's an after effect of the steroids, my body hates them. I had some oral prednisone (first time ever) for a weird rash (likely MCAS) and caught a cold from school 3 days into the course and I never get colds!
It's great you were able to wait while they did the biopsy testing especially on the NHS! We have the Skin Institute down here in NZ, where you can get lesions biopsied then wait a few hours to get the results. They provide a lovely lunch (if you're not too worried to be hungry) but it's private so you pay or have medical insurance.
It's hard not to get wound-up when there's actual visible, physical symptoms or weird stuff going on in blood tests ... it's a work in progress