A few hours ago I was on my bike and riding to the campus lounge to get my laundry. I hit an icy patch and fell pretty hard on the concrete and ice. My head was cushioned from the fall by my arm, the bone of which hit the ground pretty hard near my elbow. I'm perfectly fine, but as soon as I stood up I suddenly felt dizzy and nauseous, like I needed to sit down. I was shaking, and didn't want to move around so I stood there for a while until I felt like I could continue biking, which was a little tricky!

I'm fine, have no worries and assume this is normal, but it was something I'd never felt before. What is it called? Why does it happen?
Thanks.