I started taking Citalopram a little over a month ago. I started at 10mg for two weeks, then my doc increased my dose to 20mg.
My anxiety in general made me feel very nauseated, so I don't know if I can accurately say if the drug made me feel sick or not. I did feel a slight increase of nausea when I started the medication. I was advised to take the med whenever I felt was the best time, so I don't think it matters if you take it at night or not. I initially took it at night, and felt an increase of anxiety as well as naseau, so I started taking it in the afternoon and felt much better. You'll find the time that works best for you. I suggest taking the pill with a little bit of food, I usually eat some of those digestive cookies for babies. My anxiety made it hard for me to eat anything, so most of the time I was taking the pill on an empty stomach, which could explain why I felt a little nauseated. I am too a vomit phobic, so I understand your concern. Honestly I only felt nausea the first two days and after that I was fine. I did not throw up and like CircusSquirrel said, actually vomiting is a rare side effect. I did have a few other side effects for about two weeks, but haven't noticed any recently.
I think 20mg is a bit much for a starting dose. Try starting at 5 or 10mg, you won't feel the side effects as much if you start off lower. Just remember that everyone's experience with medications will be different, so the way I felt initially on the drug may vary from how you will feel. If you have any more questions/concerns feel free to ask!