For me, it means too much tension or weight, no matter how slight.
It happens when I wear a high bun too much, even when it is slack. The weight of the bun on top of my head makes it sore. I will then wear my hair down, in a loose low ponytail, a bun where it is not in a ball of hair, or a loose french braid. I just need to make sure there is no concentration of weight in one spot.
It also happens when I braid my cornrows too tight.
Basically if my scalp is sore, I need to let my hair hang loose.