[Supafly345]

It is probably built up fatigue. After missing a little of sleep here, or there, it can rear back over you whenever your body wants. If you are interested or working on something then it will usually be kept at bay, but sometimes it just cannot be helped. It used to happen to me at school where I would suddenly be sapped of energy, even during discussion, and would last the rest of the class. After a quick 10 min nap between classes though it went completely away.

You might have just expirienced a seconed wind type thing, where you kept fighting to stay awake so your body finally turned on again.