I was walking when i had sudden sharp back pain. it almost felt as though i hadbeen punched in the back. it knocked the air out of me. what happened?

Mnay possibilities: This depends where in the back. It could be lower back and we think of muscle, pinched nerve, jointpain. The mid back and upper can suggest lung such as pleurisy or blood clots + cardiovascular as well as musculoskeletal. With the information you have not included it would be hard to go beyond this. But since you never had pain like this before would discuss this with your physician