COVID-19 is certainly not the only illness that can cause those symptoms - COVID-19 is just far more severe than many others, and is very easy to spread for a sustained amount of time, which is what makes it more dangerous. Anyway, just because she has many of the symptoms doesn't mean she specifically has COVID-19, and whatever she DOES have is more than good enough reason for her to stay home - people don't need COVID or anything else to spread.
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I'm no expert though. I'd just call or go to the hospital and ask if you're worried.