Look up Uhtoff's Syndrome. I occasionally will run a slight temp when I get overheated, it doesn't necessarily have to be hot outside, just my body core temp gets higher than normal.

You could have some sort of infection, UTI is probably the most common culprit.

I know that in my area of the country, there is a lot of flu/cold/sinus stuff going around because of the smoke/haze/pollen count being high. I've got a horrible cold right now and am being watched for a sinus infection.

Can't hurt to get it checked out. Some people do have UTI's that are not symptomatic for the first few days.