Odd rash - any ideas?

[xise]

I've gotten a very similar rash three times more than a decade ago, and it only lasted 2-3 days and were not itchy. The occurrences were separated by years, and the only common factor was that for two of the occurrences, I drank about 1.5-2 liters of diet coke the day before the rash appeared. The other instance I think I only drank 1/2 liter of diet coke - which was very common for me decades ago. All three instances were also high stress studying for exams days (but I've had plenty of high stress days since...).

But there are probably tons of different rashes that look similarly...

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Edit: On closer examination, it appears the red spots are where the hair follicles are...that definitely wasn't the case for my rash.