It may help you to keep a food diary with the amount of Insulin you are injecting and the result of a monitor test two hours after eating. That way you will know that the next time you eat the same food what the outcome was for the Insulin injected.
Walking does use up the glucose in your blood quicker than when you are being inactive. Some people inject slightly less Insulin with food if they know they are about to be physically active. But if the activity was not planned it is a good idea to top up with something sweet to prevent glucose levels dropping too fast. When you feel a hypo coming on have something sweet too.
We all react differently to Insulin, food and exercise. When I go for a walk, I top up with a couple of mini jelly beans every 20 minutes or so while I continue walking. Other people I know need considerably more to stop glucose levels dropping.
It does get easier as your body gets used to your new way of life. You will learn (with the help of your diabetic team) how to vary your Insulin to cover the food you are eating. Your team are there to help you - so continue phoning the Diabetic Nurse as often as you need. Has he advised what level to have your blood sugar before going to bed?