All my 3 children have gone through a similar thing while they were/are toddlers.... infact my 2yo has been through it within the past month. What I find is that a cold/flu will go to their chest and the cooler night air will set off a cough... also lying down will set off a cough because (I think) the fluids (congestion) in their lungs tickles as it resettles into the lying down position IYKWIM? Anyhow after a bit of coughing the gag reflex is often triggered... and voila: vomit! ...and yes, only at night, about half an hour after going to bed it seems. After they have their vomit they usually settle well into sleep for some reason. It could also be the ear... so yes, if you see a doctor make sure the ears are checked.