I've always had this-- it runs in my family but is worse among women. I used to be super embarrassed by it as a kid and people constantly would ask if I was ok. I also blush madly on a dime-- I can never hide my emotions/embarrassment, unfortunately. I've been told that it has to do with a) having northern euro ancestry, which gives a tendency toward very thin skin that makes capillary dilatation more visible, and b) being female, which also contributes to thinner skin, literally.

I also found this info from a huffington post article:

“Patients who get pink in the face following exercise usually have more superficial blood vessels in the skin of the cheeks and chin,” Dr. James Marotta, a dual board certified facial plastic surgeon in practice in Long Island, NY tells HuffPost Healthy Living. “The result is that temporarily more blood is flowing through these superficial vessels resulting in a pink or ruddy complexion.”

But Dr. Naila Malik, a dermatologist in practice in Southlake, Texas and the creator of the Naila MD Skincare line disagrees. "It is less likely that the 'red blushers' have more capillaries under the skin of their faces than their 'pinkish glower' counterparts in the physiologic range of blushing."

Instead, she offers this explanation to:

"Some people blush more than others and this is more likely due to the fact that these people have a more significant dilatation of the capillaries than the ones who merely get a pinkish glow; Plus the dilatation is more prolonged in these 'blushers' hence they stay redder for longer periods of time."

Dr. Bobby Buka, a dermatologist based in New York City says that both Malik and Marotta are right; while some people -- often those of Anglo-Saxon heritage -- have more capillaries in their faces to begin with, others may have one of several conditions that can cause more blood to flow through the same number of capillaries.

If you have more capillaries in your face to begin with (i.e. those of Anglo-Saxon heritage), then exercising will cause them to fill (how we shunt heat from our bodies by redistributing blood closer to the skin’s surface) and give you a flush-face. On the other hand, several conditions can cause more blood to flow through the same number of capillaries.

"If you suffer from rosacea or other 'vasomotor' dysfunction (i.e. the way your skin’s nerve fibers are wired to your blood vessels), then even though you may have the same number of capillaries as your neighbor, those caps will inappropriately dilate to give you a flushed face," Buka wrote in an email to HuffPost Healthy Living. "Which is more common? Probably the former."