Looks like you got a syncopal attack due to dehydration-electrolyte imbalance. Take more water. Take a water bottle woth you. Most important, is whenever you have such symptoms,  do not try to resist them, but just take a pause and sit or better lie down wherever closest possible as soon as possible. This should stop once your body adapts to it. If this doesn't work, then consult online for better history to rule out other possibilities in your case. Keep upurself hydrated. In case this happens more frequently or while sitting/at rest, you should get an ecg followed by a holter monitoring, if required.

Need detailed history and evaluation