Advice for ftm please!

My son is 2 weeks old and yesterday he had a temp of 99.9. I was told not to worry until he’s in the 100s. I just kept an eye on him. Then he vomited about an ounce of milk. He seemed fine the rest of the day though. Now since 8 am this morning he’s been fine one min then the next crying cuz of his stomach it seems? It’s noon and he won’t sleep because it hurts him. I’m not sure though. I don’t know whether to take him to the ER or just wait it out and see n if it gets worst to go?