I cannot remember when was the last time I have this feeling. I think it was a year ago in Thailand during my NS time. I cannot reveal about the things I did in Thailand because if I do and you saw, I have to kill you... with cheese...
After 15min of rest, I took the train again. Upon reaching Jurong East MRT, the same feeling came back. I quickly get out of the station and rushes to the toilet to release myself. Then I went to buy stuff to drink and eat.
I suspect it was due water and fatique issue. The previous night I did not really have a good sleep. And the next morning I only have a sip of water and no breakfast. In Army, we were always trained to, and subsequently train soldiers to water parade first thing in the morning and before lights out. This is to hydrate their body for the day and for the next day (Night water parade).
Morale of the story, if you foresee yourself having to attend a morning parade (Defined as standing prolong without much maneuver. e.g standing in the train) after a bad sleep the night, please do yourself a favor by drinking enough water before set off. And drinking enough is subjective. So, we always say that drink beyond your point of thirst. However, if the journey is long, do not drink too much as you might find yourself looking for the nearest washroom during the journey. My valuable advise is.......drink a sip is enough, period.