I'm going to go against the grain here and say it may only be related to teeth. Both my boys got temps around the 39 mark when teething, it only last 24-36 hours and was brought down with panadol or nurofen.

I had a lot of health professionals tell me that a temperature was not related to teething but based on personal experience, I believe that it is, well, at least it was for my boys anyway, because as soon as the tooth cut the temp went away. No coincidence there