Has my 4wk old baby caught a sickness bug?

my daughter was unwell 2days ago and was trying to be sick but only bought up a small amount of bile. Now today my 4 week old son has bought up whats looks like all the milk he has had today and keeps crying out as if he has belly pain. He is sometimes a little sick after a bottle but he bought so much milk up today i wondered whether he had caught a bug? does anyone know of anything i can do to help him? all answers apprieciated! thankyou