Toddler not eating and drinking lots

My 20 month old is refusing to eat at all today. He is drinking loads and loads though.

Started last night he cried every hour getting super upset, thought it was constipation but he went okay this morning.

He also vomited at 6am- he hadn’t had anything so it was like pure mucus and breast milk from hours earlier. Getting a bit anxious is this normal? Is this a stomach bug? No poo or sick since, just sleeping and not eating