He swolled a few pieces and seemed fine for I think a couple hours. And about a hour ago, he starting vomiting hard, a lot in about 30 seconds time. I sat in the bath with him and he's thrown up twice since then. He is now asleep. He has no fever, but looks flushed, I'm guessing because of the vomiting. Should I be worried? Is there anything I can do besides calling the doctor or taking him to the ER?

What did he swallow?

What is an 18 month old doing with gum?

Gum shouldn't induce vomiting. Most likely it is a virus.