What are the strange bruise like marks around my neck?

About a month ago my family and I noticed a strange mark around my neck that looked like a bruise. It is not a hickey and I've gotten that enough times. It does not hurt like a bruise nor change pigment when poked. It goes perfectly around the front side of my neck fading in the middle and darker on the sides, also it is perfectly flat on the sides. It is about an inch from where I grow facial hair and an inch above where a lot of my shirt lines are. Also a few large pimples broke out and I have never gotten them in that area before. I don't tan very often, I wear a shirt to bed about every other day, I don't wear a necklace. I'm 20 5'10 255lb not obese. I nor anybody I know including a nurse istant and nurse practitioner can accurately tell me what it is. It just started fading on my right side where it was darker yesterday. If anybody has any info that would be great.