There are many potential reasons for feeling lightheaded at different times during the day.

Let's start with the lightheaded feeling you get when you stand up after kneeling down for a short period of time. This is very common, especially in people over the age of 60. When you stand after kneeling, your blood pressure temporarily drops.

The drop in blood pressure causes you to feel faint. Usually the lightheadedness will pass within several seconds. But you do need to be careful, because it could cause you to fall.

Medications may be contributing to the symptoms. Blood pressure medications, water pills (diuretics) and several other drugs can cause blood pressure to drop significantly when going from a kneeling to a standing position.

The faint feeling after walking worries me a little more. This could be related to a drop in blood pressure, but generally blood pressure doesn't fall during exercise unless you took a bunch of blood pressure pills just before you started your walk.

Sometimes feeling faint while exercising is a symptom of coronary heart disease or some other type of heart problem. Your doctor needs to know about this symptom.

Once a heart problem is ruled out, another potential cause of feeling faint is low blood sugar. This occurs quite often in people with diabetes who take their diabetes medications and don't eat a snack before going out for a walk.