Agreed, not cotton fever. Trust me, you'd know. My temp has been 105.3 during an episode of cotton fever. They key word here is fever, man, an intense, miserable experience lasting maybe 6-8 hrs. Your teeth chatter uncontrollably, your limbs shake, knees hurt, nearly impossible to walk or even stand...
Clear, measurable temp increase, feeling chilled/feverish are often first signs.
The very 1st sign is no high or euphoria resulting from the injection of your shot (though there are other reasons you may not feel buzzed, and so isn't a clear indication)
I had perhaps 7-9 very unpleasant episodes of it over 3 years timespan, I'm now very conscientious about hygiene & technique. Bad stuff.