If it's any consolation, I've never been underweight and have had all of these symptoms. Go figure.

Anything that increases blood circulation should help. If you have to sit for long periods, make sure to get up and stretch frequently; tap your toes, stretch out your fingers, etc. Sometimes just staying hydrated can be a great help, like you said. Tea is awesome in this regard because you can drink it hot and it temporarily warms you up, or at least it does for me.

Vitamins may or may not help because if you're not eating in the first place your body won't digest or absorb them. B-U-T, it can be a good excuse to get yourself to eat at least a little bit of something, if only for the sake of taking vitamins. Or, drink some kind of sports drink/shake that has vitamins in it. Otherwise they'll just kind of sit in your gut and not do anything.

Try to be kind to yourself, even though it's not always easy... Someday you'll be able to tackle the source...