About 3 months ago my DS made himself sick whilst sat in his highchair eating tea. I wouldn't have believed it if I hadn't seen it. He had food at the very front of his mouth and started coughing and retching, spat out the food and continued to retch until he was sick. He did go through a phase of imitating coughing (between 5-8 months) but never anything like this.
Tonight he sat in his carseat and stuck his fingers into his mouth and retched until he was sick. There was nothing in his mouth, he had his teeth brushed 5 mins before we left the house. I was looking at him and talking; he was babbling back, whilst he was talking he stopped putting his fingers in this mouth and didn't cough or act in any way that suggested he had something in his mouth (fluff, hair etc). He then put his fingers back in his mouth and this time (about the the 5th time he had done it in 5 mins) succeeded in making himself sick.
We were off to a fireworks display but turned round and came home. I'm at a lose as to why he has started doing this. My MIL noticed he was sticking his fingers into his mouth and coughing this week and I had seen this as well but had put it down to chewing his fingers as he has been teething.
DS (22mo) went through a very short phase of putting his fingers down to the back of his throat and making himself retch a couple of months back. Not totally sure what caused it - I thought it might be something to do with his molars coming through but it's possible he was just exploring what he could do. He seems to have stopped it now thank goodness - hopefully it's just a phase for yours too!