What Causes Bruise Like Mark In The Middle Bottom Of Neck?

For the past week, I've had a strange bruise-like mark in the middle-bottom of my neck, right above where the collar of a t-shirt would fall. It is not tender to touch, but it doesn't seem to be going away. It seems to be grey with tiny black dots. There also seems to be a light grey ring around the base of my neck, but that could be due to a necklace I wear daily. Any ideas on what this bruise-like thing could be and how to get rid of it?