Is there any chance you might have celiac disease? Depression, lack of appetite, IBS - your symptoms suggest that celiac is a possibility - although admittedly, there are many conditions that show those symptoms. Just something to look into.
In the meantime, as you're a vegetarian, you could try easily digestible soups & juices as others have suggested. Also try cooking a bowlful of quinoa or brown rice - both are easy to eat & digest - and add fat like olive or coconut oil or even butter to keep your calories up. Hope you get better soon :-)