One of our kids comes to our bed occasionally at night. He just seems to have an emotional need to do that once in awhile. We don't sleep as well on those nights, but he needs, so we're there for him. We let him in our bed, because it's what he needs.
If your son threw up on Sunday, I think it's safe to say that he's not exactly fine now. He's probably better, but he might still be a little under the weather or just tired (from being sick). He could also be a little bit emotional from throwing up. That's very upsetting for some kids.
Unless this is becoming a regular thing, I wouldn't analyze it too much or try to "fix" it right away. We all have bad nights now and then. Kids, too!