Why do I cough up phlegm or liquid after almost every dinner?

I don’t have a very big appetite and if I have a roast, with one slice of meat or one potato extra, (3 slices of meat and 3 roasted potatoes and a piece of roasted pumpkin); I end up throwing up after dinner. It’s not vomit either, it’s milk or gravy or phlegm usually.
Why do I cough up phlegm or liquid after almost every dinner?
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