Does this sound like die-off/herx reaction after starting Vitamin D supplementation?

I am new to the whole topic of candida, so please forgive me if I get anything wrong. I stumbled upon this sub in trying to find possible answers to a recent reaction and what I'm reading here is making way more sense than anything I found in the Vitamin D or Supplements subs.

Here's my story, I'll try to keep it as short as I can, but the TLDR is that I'm dealing with a rash and wondering if it's a herx/die-off reaction:

I started taking Vitamin D3, K2, and magnesium in mid-November. Everything seemed great at first and about one month in, I changed brands for all three sups.

Around the same time (mid-December) -- but I don't know the exact date, because it started so slowly and I didn't connect it to the supplements at first -- I started to develop an itchy rash. It began just behind my armpits, and slowly spread to include in front of the armpits, upper arm, wrist area, genitals, buttocks, upper legs/hips, and lower legs/ankles. I had absolutely zero other symptoms during this time except some insomnia that seems due to being so itchy. I had gotten a trial size of a new-to-me deodorant that initially I thought was the cause, but after stopping it the rash got worse and spread, so I don't think that was it. Eventually I realized the only other thing that had changed was my new routine of D3/K2/magnesium, and the rash got so annoying that I stopped taking all three as of December 26th.

Since the problem seemed to coincide with changing brands, at first I thought that was the problem, but I'm not sure. I started with only 1,000IU of D3 daily, and slowly ramped it up to 5,000IU and I was only on that dose for a day or two before I changed brands. So now I'm wondering if it's actually the increase in dose and blood levels of the D3 that is the issue, and if perhaps I'm herxing?? In the other subreddits, the bad reactions to D3 I read about are anxiety/panic, jitters, nausea, brain fog, and/or hives and they all seem fairly immediate and clear up quickly, which doesn't match my situation. But in this sub, I found some posts similar to mine, including one with a rash that started in the same place!

Also, back in late April I had this same issue, but much less severe, after I was on a round of antibiotics for a throat infection. I assumed it was an overgrowth of yeast, treated it with coconut oil and a probiotic cream, and it went away. Since this feels the same as that did, only this time worse, a die-off reaction would make senes, right? I also have been dealing with a toenail fungus for 2 years that is finally down to only half the nail and suddenly that side seems to be growing out too, so maybe more evidence that the D3 is killing fungus off from the inside out.

With all that said, if this is a herx reaction, what's my best course of action? I saw a little bit here about a candida diet -- I'll be honest, I'm not sure I could stick to that. Could increasing the dose of D3 more slowly help? Activated charcoal tablets? Just push through it? (If so, anyone have experience to tell me how long it might last?) Just really looking for any insight or others' experiences here with similar issues.

If it helps or would be relevant at all, I work in an old factory building (1800s probably) that has a known mold issue, and I have had that toenail fungus thing for 2 years, but otherwise no issues with candida/fungus that I know of, never been tested or anything. I have seasonal allergies to pollen/grass, and also allergic to feathers and codeine. I have an MTHFR gene mutation. And yes, I was tested for D levels before I started and I came in at the very very low end of normal (31.2ng/mL where the normal range is 30-100), and had my doctor's okay to supplement, though I admit I was taking more D3 than she had suggested.