Hi guys! please help me. I noticed a patch (circular form) on the shoulder of my yorkie and all of the hair has fallen on that area! The affected area of the skin is a little bit redish with blemishes and it is in a circular form.
Does anyone here experienced this on their yorkie? What could this be??

I'm so worried. Though he has not experience appetite loss and he still is very energetic.
Any ideas guys on what this could be?
Thanks!