Sudden fever in minutes?

So yesterday I woke up with a sore throat and started felling achy as the day went on. No big deal figured it was a cold. Today was the same thing cept a bit more muscle pain than before. My throat doesn't as much as yesterday though, but now im worried about this sudden change in temperature. I took my temp about 2 maybe three hours ago and it was 99.1. This was around the time the aspirin I took started to wear off. An hour later or just about it went to 99.3. couple minutes later its 99.5 and now its 99.8! Idk if this is normal or not. Is something wrong with me or my thermometer. Should I check this out or am I being paranoid and should just sleep it off?