Again last night. He slept 7-11 (not quite as long a stretch as the past couple of nights but he'd pooed which I think woke him). I fed him, he went to sleep fine and then woke again at 3.15. I fed him then and after this feed he lay in his cot doing the same thing - grunting really hard, squirming and straining. This went on til about 6am when he finally started to let out the odd loud hard cry. We then picked him up and tried to comfort him when he continued to grunt/squirm etc and occasionally cry and then I fed him at 7am - and he was a little bit sick after this feed, not sure if that's relevant.
It seems to be if he feeds in the early hours of the morning, this grunting/squirming/crying thing happens and goes on for hours - it's doing my head in!!!