What Causes Dizziness While Walking In A Cancer Survivor?

I have had 3 episodes over the last 2 to 3 months where I am out walking (with dog) and I suddenly feel very dizzy and in the most recent case could not continue standing and fell over. I took my blood pressure on returning home and it was normal (118, 82, pulse 60). I am 72 years old, male, retired and reasonably fit. I do not have diabetes. I have had radiotherapy for prostate cancer in 2011 and operation for bowel cancer in 2014. Sometimes when I get up to visit the toilet at night I feel slightly dizzy for a few seconds. My medication is 10mg of amlodipine which I have been taking for about 10 years for mild hypertension.