Probably got jolted out of deep sleep, like people said, or maybe your blood pressure was slightly low, which can happen when you've been lying down and stand up too fast. Basically what people call a head rush. However, when I'm woken up I stagger around like I'm on a moving ship - I literally slam into walls. Besides being annoying for me and a source of comedy for those around me, it's not significant. It's just something that happens to people.