My dd does this as well, and also sometimes if she coughs for too long (so flu season is a joy at our house!) She also gets car sick, and vomits after going on the trampoline (I, glad we worked that one out before we bought one). The doctor has said she probably just has a sensitive stomach so eve accepted that and have been trying to teach her her limits when she gets upset and how yo calm herself down before she vomits. I have thought recently about getting it looked into further but haven'r done anything about it.
I apparently did this unitl I was about five - f I didn't get my own way I would throw up. Now I am not saying your DD is doing this now but rather that she has a sensitive gag reflex which is triggered when she cries or is upset. With time she may learn to use it to her advantage (as I certainly did LOL!) but it may not be anything too serious. But if you are worried take her to a GP, just for peace of mind
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