Winter has Come..! found news article related to fever..
 

Feeling a little heated? It’s typically nothing to worry about ― especially if you’re sick. However, other issues can cause your temperature to rise. A normal body temp is typically around 98.6 degrees Fahrenheit (37 degrees Celsius) but can vary slightly for each individual, and even fluctuate depending on the time of day.

“It can be lower in the morning and higher in the late afternoon and evening,” said Michael Hall, a physician based in Miami. But when your temperature gets to 100.4 degrees Fahrenheit or higher, and lasts more than a few hours, you’re getting into fever territory ― something that can be caused by a number of conditions.

“Most of my patients understand that a fever is a common symptom of cold and flu,” said Christopher Dietz, an area medical director at MedExpress Urgent Care. “However, what some people might not realize is that a fever isn’t always just a sign that you’re coming down with something.”

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I hope you all well~ take care.. :)

I would say any temperature over 100 especially if accompanied by symptoms such as pain, or any other worrying things (not just an average feverish cold) warrants a trip to see the doctor.
With very young children and babies, talking to a doctor quickly, even over the phone, is advisable.

But yes, for all of us who are normally healthy, temperature can change by the hour, and can go above normal even just after strong exercise.
And it can drop a LOT in the middle of the night.