First of all, is this a chronic issue or a acute issue? If it is a chronic issue, then it is likely just to be your perception of relative warmth and not actually a serious problem.
However, if your testicle feels acutely warm all of a sudden (or recently, in the last few days) then this could be significant. It would be especially significant if there were other symptoms, such as redness of the scrotum, pain in the testicle, or pain or discharge from the penis. There are multiple infections that could cause those types of symptoms; some of these are sexually transmitted infections and some of them are not.
The best thing to do, if you are experiencing some of these symptoms that I mention, would be to get in to see your primary care doctor
. They will be able to take a good look and help you decide if this is something that you need to be worried about. They may want to take some blood or urine tests to help sort things out.
If you have pain in the testicle that is very severe, you shouldn't want to see your doctor; instead, you should go right to the emergency room.