i vote calpol!!! could be anything really, does he have a temperature? (i guess not if he felt cool). could be growing pains, sore tummy, very likely to be teething troubles, maybe he is just feeling lonely tonight!
i would honestly go for calpol if he isn't being sick or nothing else seems wrong, sometimes it really is just 'one of those things' and you will wonder what was going on tomorrow. we found gaviscon helped elijah but that was ongoing from when he was teeny so i don't think reflux randomly starts at jamies age (could be wrong but it seems more likely to be connected to sniffles or teeth).
so if you have any in the house, calpol or baby ibuprofen...even teething gel/powder just to see if it makes any difference (or even just distracts him out of it)