Is 37.9 degrees Celsius a fever?

It's a little higher than the average temperature which is 36.8 degrees, but 37.9 just means you have a slight fever and your body temperature is just a little higher than normal, if you done some hard core excercise a while ago it may affect the temperature as well.

It's nothing to worry about though, I suggest you take some Vitamin C pills or some fever reducing medicene to get back to the normal temperature!