My dog is developing bumps on her neck area & the top of her head. Do you think she could be allergic to the metal in the collar?

I have a four year old German Shepherd that wears a prong collar. I have consulted with a vet and behaviorist to be sure that it is fitted properly. She began wearing a prong at about age one and after realizing the restriction, hurt herself once or twice and now just jumps straight up and down in the air when she is out of control for her fear of other dogs. (Trust me...something I have worked on) but the point is she never hurts herself with the prong collar.

My concern is that over the last six months, she is developing small bumps on her neck and the top of her head. Of course she only wears her prong collar on her walks when I have her on a leash but I'm thinking she is allergic to the metal. Is there another material you sell that may solve that problem...have you ever heard of this before?

I feel sorry for her since she has these bumps developing all around her collar area. Any ideas would be helpful

Please let me know if you have another collar that might not cause her allergies.

Thanks