How long have you been on lithium? I found the first 2 months I was extremely sick from it. Possibly also a bit from the wellbutrin(and some weeks a lot of alcohol) but mainly it would happen a little while after taking the lithium. I was on 600. That included nausea and almost every night vomiting. I slept with a bucket because I would even wake up in the night if I went to bed right after. My levels were fine in this time though and it did start to get a little better with time.

Hope it doesnt continue but if you are worried its probably not a bad idea to get your levels checked