This was what I had experienced before a mild few times: During Boys Brigade's (10th company, Chinese Baptist church) hot sun march in Butterworth army base years ago, which thanks to God a captain recognize it from my face before it got worse. Another vivid memory was a few years back in South Amboy when Nadia and I went biking and to catch up to her electric bike on my mechanical bicycle, I exerted so much strength to a sudden rest and a foolish sitting on the chair caused it.
When this happens, blood pressure dropped and not enough blood brings oxygen to the brain, causing dark spots vision building up. Do not sit or it would block blood flow from legs. Walk arouns keep leg active, drink water to hydrate blood volume.