Blisters from back face of watch

I’ve recently started getting blisters in er the face part of my watch. The first looked something like a red spot but this second one hurt like a cut and looks like a blood blister. I’ve had an iwatch for 4 years. I upgraded from a 2 to a 3 about 5-6 months ago. These blisters just started happening in the last month. Why is this happening? I keep it clean and don’t wear it in the water or anything so I’m confused why it’s irritating my skin so badly when it was fine for over 3 years.