What is considered a fever in newborns?

Any advice would be very helpful. My little one is almost 4 weeks. She's been feeling warm all over for a couple of days. I checked her temp. by rectum & it was 99.8. I also checked it with a temporal thermometer & it read 98.4, also under the armpit was 98.4. Which is most accurate? Should I be worried? Could it be because she's hot?