infant not keeping weight down

MH or anyone can answer this question, my sister in law her 3rd is about 3mos old and at first the baby is keeping weight down but when the baby is going on two weeks old, everything kind of spiral downward, the baby is breastfed, she's got 2dogs, and now she's going thru a divorce....so the stress is really high at this moment, there is no known health problems or hereditary related, but everytime the baby takes a feeding either he throws up or poops it all out, the baby is extemely underweight, and is currently at O'Reily hospital, so i dont even know if that is a good thing....and they're doing all kinds of tests on him....any ideas of what could be causing this...and im sure he has his vaccinations as well assuming because she's pro-vax Tweet