That needs to be checked out. It could be a fungus. Depending on what it is, there will be a different cure. Please consult a dermatologist ASAP.
Otherwise, try a hot bath with chamomile teas bags and oatmeal to soothe the skin, and put aloe vera on afterwards.

Damn in a hot tub
Well This is most likely chlorine rash you get it from a off set in chlorine intake on ur skin. i had a mild case when we got r first hot tub. hers what i Did:
First i freaked out (lol don’t do that step)
2 i rinsed my body of with cool water and soap
3 soak in a tub of fresh water for 10-15 min
4 drain and refill
5 Add Oatmeal (no joke this works)
6 soak for a bit then when u feel like u’ve soaked long enough get out dry off and air off
7 after 1-12 the rash will be gone (due to how good u soaked)

That sounds unpleasant see a doctor