Also starting mine, We both have a Vitamin D deficiency and the same symptoms: right before I sweat, my back (and maybe upper arms, thighs, and belly) start to feel little needles. There's a sharp pain for a second and then I feel a need to itch it. I've been scrolling through google for so long looking at anything for a solution, but nothing. I see so many people in a similar situation and read long forum posts. I've gained weight and quit tennis because the pain I get is not worth it, and it's embarassing when you move your body in a weird way in public and have to scratch it really hard. I'm so frustrated, and I feel like when I tell people close to me about how much these seemingly minor symptoms affect me, they don't take me seriously. I'm scheduling an appointment with a dermatologist and immunologist soon. This situation is really similar to a while ago when I used to get food stuck in my throat. I sat there for almost an hour scrolling and reading, seeing so many people with similar symptoms but no answers. When I ate something, it would randomly feel like a lump near my spine inside of my ribs, or sometimes in my lower throat. Went to a doctor and all of a sudden I was diagnosed with Eospinophilic Esophagitis. Luckily, now if someone looks that up GERD won't be the only thing suggested to them. If you haven't gone to a doctor yet, you should. Since the thing I think is causing my problems is histamines, a visit to the dermatologist and/or allergist/immunologist seems like a good idea to me. I know this sounds stupid but I know a few friends who were in a similar situation and were finally diagnosed when they went to the doctor. I know some people are scared of doctors, but they should at least be more than a last resort. Hoping I get lucky again and there is some unheard of but common condition that makes me allergic to sweat or something, and hoping I can treat it to. I'm going to start Vitamin D supplements tomorrow, too. Sorry if I wasted your time with this rant, and I wish you well on your needle-sweat journey.

TL;DR: Go to a doctor