Christmas lights plug feels warm

[XmasStag]

I had Christmas lights plugged in for about an hour. When I plugged them out, the plug felt warm. Not hot. Just slightly warm.

It was a double socket and the appliance and plug near by was cold to the touch.

Would the Christmas lights be bad?

Or will I plug in something else tomorrow and see if that heats up as well and see if it's the socket?

I often find that when I unplug something, the plug is warm. I asked an electrician friend and she said it’s nothing to worry about.