Dark red streak on back of neck

Hi. Just wondering if any of you have this symptom and know what causes it. No traditional or alternative health practitioners I've seen has any idea what it is.

For years, I've had a red streak on the back of my neck that goes up into my hairline. It seems to get darker when there's more inflammation in my body, as when my platelet counts are low. It's under the skin, not like a surface rash, but looks more like histamine pooled under the skin or distended blood vessels. It sometimes itches, but never bleeds or swells. It changes size and is more or less noticeable at different times.

My mother had a smaller, very dark red rash in the same location. I think it's some kind of autoimmune symptom, but don't know of what. I've asked a variety of practitioners about it, including hematologists, allergists, dermatologists, internists, herbalists, and naturopaths.

Any insights would be appreciated. Thanks!

Cathy

I also posted a similar note on the Histamine Intolerance Facebook group, and got six responses in 15 minutes from other people who have the same symptom, but no diagnosis!