Why did I suddenly get a high fever?

On Saturday morning I developed chills and body ache followed by intense fever. My heart was skipping beats fast during the fever. I also felt naseaous. This was going on the whole day till night got a bit better as i tried to cool myself off. Yesterday I recovered but still has some warmness and headaches. Today I'm completely better. What I don't understand is what on earth happened? Was if flu, covid? What else?

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1 y
* was it

I probably had like 39 or close to 40° fever
Why did I suddenly get a high fever?
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