My last unbraked trailer did exactly the same. It was probably because I overgreased every trip. The test is the temperature of the hub, not the tyre. Generally the tyre will run warmer than a car tyre since it is loaded up pretty heavily.
If the hub is only warm after, say, thirty miles, all is well.
As a matter of interst, I always stop after about ten miles and check the temperature of both tyre and hub, just to be certain. Any variation from the norm, or accros the trailer indicates an impending issue.
It is good to get into the habit of feeling the hub at every stop. Gives you confidence that all is well. You will then learn what is normal.
Also, don't dunk the trailer if the hub is hot. That would draw water into the bearing.
Hope that helps.