Getting out of breath when you walk could be caused by various things.
It could be due to being physically unfit, no exercise in a long time, sitting a lot, less activity, etc.
It could be due to medications you are taking.
It could be a lung problem.
It could be due to wheezing.
It could be due to smoking.
It could be a heart problem.
It could be due to being overweight.
It could be due to being a hidden hyperventilator, few doctors recognize this.
It could be due to anxiety.
I am currently in process of finding out why I am short of breath. Next step for me is to have a stress test to rule out heart problems. I know I am physically unfit, have been unable to exercise the past 4 years due to health problems. Sitting is not called exercise for good health and I have had to do just that way too long.