Was on 15mg lisinopril then 10mg then 5 mg daily then 2.5mg every 12 hr. Took 5mg in morning and was hypotensive 24hr later. Why?

You don't need it? Possibly your blood pressure is not really high any more or you were started on treatment on the basis of only one or two readings. Sometimes, if the heart doesn't pump as forcefully, thyroid level is low, you have lost weight, or other reasons, the blood pressure drops. Being hypotensive can be dangerous (falls, possible strokes, or heart attacks) at your age. Ask your doc if you should stop rx!
Not sure. 5 mg of Lisinopril generally won't make normal people hypotensive. This may be related to your age. I would suggest that at your age Lisinopril does not make you feel any better and it is very unlikely to make you live one day longer. Therefore consider stopping it and all other meds before they hurt you.