I Frequently Get Light Headed When Getting Up From A

I frequently get light headed when getting up from a chair or out of an auto. Today, I was taking a nap, was awakened, walked maybe 150 feet feeling just a little light-headed, and then suddenly my knees buckled and I sat down on my rump. I got up and felt fine. I am diabetic on insulin pump, so took 15 g glucose tablet and drove home (15 minutes). I checked my blood glucose . It was 100mg/dL. I am 80 years old, completely ambulatory and have Stage 3 kidney failure and hypothyroidism. I visit Drs. very often (usually each 3 months for each condition). This is the first time my knees have completely buckled, but they have felt weakness at times when light headed. Any advice?