What was the cause of this fall and what is the prevention?

I was walking on the sidewalk with the mail. But at the same time I had my head turned to the right as far as it would go to try and see the new house up the street. I fell down. My nose was at grass level; my left knee was on the pavement. I got up as fast as I could in case anyone was looking and walked back inside. The skin on my knees burned like fire! And the left knee had swelling. There was no break in the skin and no blood. Weird. That was Saturday. Today, Sunday, each knee has a tiny blue bruise and they feel fine. Is this a balance problem? How do I prevent this? TIA !