Unless you’re the most serious of dry shampoo addicts, most of us wash our hair reasonably often (even though the styling that often comes with it can be a serious pain). But no matter what, it seems like sometimes hair-washing falls to the bottom of our priority lists.

Of course, when you don’t have time to wash your hair, you can end up with a scalp full of grease. And unfortunately, that’s not all you can get from skipping washings. Ever notice how your scalp kind of hurts when you haven’t washed your hair in a few days? It’s not just you—greasy hair pain is a thing, and there’s a legit, scientific reason for it.

If you don’t wash your hair for awhile, oils that your scalp produces naturally accumulate around your hair shaft, promoting the overgrowth of yeast on your scalp, explains Joshua Zeichner, M.D., a New York City-based board-certified dermatologist and director of cosmetic and clinical research in dermatology at Mount Sinai Medical Center. This can lead to skin inflammation which causes redness, itchiness, and scaliness. “In some patients, inflammation may manifest most with an ache,” he says.