Kids this age fall a LOT. My son constantly had a bang or a bruise on his head until just a few months ago. He gets more clutzy when he's tired. I think it's normal, though it is sort of horrifying how often it happens :)
Just do your best to eliminate any easy causes - make sure her socks have sticky things on the bottom so she doesn't slip, or that she's wearing shoes. Keep clutter to a minimum so that when she falls she doesn't bang onto something, or trip over something (this will happen no matter what you do, but you can try and minimize it). Remind her to slow down if she's running.