I have had two children that "spit up". My youngest was extreme. I knew something was not right but the Dr's and nurses thought she was fine. She would throw up and everything would need to be changed, me, her, the bed, etc.
They best advice I received was that if it ever looks a little green go to the ER. (It was just a light greenish hint to my daughters) We caught my daughters volvulus at 2 weeks. She spit up alot, was gaining weight, because she was weighed before the spitting up, and looked healthy.
Volvulus is the malrotation of the intestines so basically she was getting very little milk through her system. It was caught in time, due to the lactation consultant that I talked to the same day we took her in. If left undiagnosed it can lead to part of the intestine being strangled... which means having a bag for excrement the rest of her life. She had surgery within 4 hours of arriving at the ER- quit scary.
A mother's instinct is often correct!