99.1 temperature

My little guy just turned a month yesterday and has his 1 month appointment on Monday. He was feeling a little warm today so I took his temperature under his arm and it was 99.1° F. He’s only wearing a light long sleeve onesie and his diaper so I don’t think it’s from him having too many layers on. I called my pediatrician but they closed early today (just my luck!) so I was wondering if a 99.1 temperature is something I should be super worried about or what I should do! Thanks in advance (from a first time mom)!