I think it's just heat rash. Here's what I found on this site (oddly, an Australian doctor site... lmao, just noticed that) but it describes what I have to a T.

HEAT RASH OR PRICKLY HEAT



What is heat rash or prickly heat?
Prickly heat, or heat rash, is a harmless but very itchy skin rash with small red spots all over the affected areas. It is common in Australia in hot and humid areas.

What causes heat rash?
The rash is caused by a blockage and inflammation of sweat ducts during times of exposure to heat and high humidity. These blocked sweat ducts cause sweat to escape into adjacent tissue thus producing the itching and prickling sensations associated with the condition.

What are the symptoms of prickly heat?

Tiny red spots, which usually appear on areas of the body where sweat collects, such as the armpits, back, under the breasts, chest, groin, crooks of elbows and knees, and the waist.
Irritating itch and prickling sensation.
Redness and mild swelling of the affected area.

What you can do:
Treating heat rash is simple and usually does not require medical assistance. Other heat-related problems can be much more severe.

Wear light, loose-fitting soft clothes. Avoid clothing that traps heat and moisture. Natural fabrics such as cotton are best.
Stay in cool or air-conditioned, or well-ventilated environments. Use a fan if necessary.
Take frequent cool baths or showers. Dry yourself thoroughly and dust bland types of powder (such as unperfumed talcum powder) on body creases and affected areas.
Apply calamine lotion or prickly rash lotions to soothe the affected areas, but avoid using ointments or creams — they keep the skin warm and moist and may make the condition worse.
Do not scratch.