Pink patch on dogs lower lip

Hello everyone. Just wondering if anyone can tell me what this might be on my 4 year old male chihuahuas lower lip. Obviously if it doesn’t clear in a couple of days I will take him to the vets.

It came very suddenly (in a matter of a few hours) and doesn’t seem to be causing him any pain or discomfort. It’s not raised and is the same texture as the rest of the skin on his lip. He hasn’t been in contact with anything he shouldn’t have been (that I know of, he rarely leaves my sight) and hasn’t eaten or drank anything out of the ordinary or that could’ve been hot enough to burn him. Any reassurance would be very much appreciated. Thanks