What is wrong with my neck?

I’ve been getting these strange acne-type marks on my neck recently. They are basically large pimples but they don’t seem to contain any pus. They just look like a mound of skin. It’s confusing because I don’t have acne anywhere else (other than the odd spot on my face, rarely). I only get one at a time and I’ve only had two or three of these “things”. Does anyone know what it could be?