I get nausea with my ibs with bowel movements. The pain gets so intense that my mouth starts watering, I get nauseaous and I began vomiting uncontrollably all over the floor unless I can reach for a trashcan.

Well that's pretty brutal. My nausea is not associated with bowl movements and i've never vomited because of my condition.

I use promethazine for my nausea but it kind of knocks me out. I am not a dr but if you go to a health food store or your dr they maybe can recommend something that will help you as I get nauseated for other reasons as well, you might even think about getting a second opinion, I know a friend of mine kept getting nauseated and her dr told her it was ibs and she got a second opinion and it was her gal bladder and something else. I am not saying that your nausea is not related to your ibs, like I said, I am not a dr but it is always good to get a second opinion and if you do throw up alot you can become very sick and dehydrated.
Good luck!