I started to develop a rash very similar to what you are describing when I was also about four weeks out from surgery.
I suspected it might have been this: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6558236/
Fortunately I was able to clear my rash with just slight modifications to my diet. I added just the tiniest little bit of carbohydrates to my intake with some apple sauce, sliced banana, etc. Previously I had been very, very strict with no carbs for approximately six weeks.
Good luck. -Neil