I had an eventful day. I went to the gym deciding it's not a special day. I did one set of moderately heavy squats (160kg), bent down to pick up my rest timer and and began teetering around and felt awful. I put my head down below my heart and still felt awful so dropped to my knees with my forehead to the floor. 30 seconds later I was still on the verge of fainting and then a fellow member came over to ask if I was OK. I said yes, it was just a bit of postural hypotension which'll resolve shortly. But it continued and continued with any attempt to raise my head causing dizziness and the wooziness. After 15 minutes I went out into reception to lie on the chairs, various blokes came in saying it was my way of getting the 'kiss of life' from one of them, offered my a drink etc and then 30 minutes later I recovered sufficient to pack up and drive home, where I tucked into my birthday chocolates! I didn't feel A1 for a few hours.
I was a bit dehydrated so had lower circulating volume and considerable intentional weight loss contributed too.