This could be an allergic reaction. It may help if you write down the things you've done to determine which could be the trigger to this allergy. If allergen is not known, patch testing is recommended.
Miliaria or sweat rash, on the other hand, is a skin disease marked by small and itchy rashes which is common in hot and humid conditions. It occurs when the sweat gland ducts get plugged due to dead skin cells or bacteria. The trapped sweat leads to irritation, itching and to a rash of very small blisters, usually in a localized area of the skin. These rashes may occur in the face, neck, skin folds, areas of the body that may rub against clothing, such as the back, chest, and stomach.This can be prevented by avoiding activities that induce sweating, using air conditioning to cool the environment, wearing light clothing and in general, avoiding hot and humid weather and taking frequent cool showers .
If this persists, it would be best to have this evaluated by your doctor for proper management. Take care and regards.
Try researching Molluscus Cantagiosum (http://www.aad.org/public/publications/pamphlets/viral_mollscum.html)
There is a possibility you can have this, as it can be passed at swimming pools, in locker rooms, skin to skin contact, etc.
It can be treated with an acne medicine that basically causes your own immune system to fight the virus.
I play a lot of sports and got this, and noticed the bumps don't really itch and only get red during heat/sweat/rubbing, so I figured I'll inform you of this virus since the 1st doctor I went to misdiagnosed it.