Dehydration can cause stomach issues and many other problems. Try to stay hydrated at least or you will have further issues to deal with.

Being depressed means your brain is in low gear and it can affect all of your systems. I always lose appetite when I get really down. It will get better as the meds take effect, hopefully. In the meantime try to eat easily swallowed things like yogurt, pudding, ice cream, even milk shakes. Make sure to drink at least a quart of plain water a day, as that is the minimum to stay hydrated. (A quart is 4, 8oz glasses or about 6 coffee cups ) Not that much really.

Do go back to the ER if you have further issues. You MUST force yourself to drink and eat if you want to not be on IV to re-hydrate.