I would hope they wouldn't get mad at you! They should understand that you're trying to help yourself get better...if you were anorexic I don't think it's very likely you'd be so interested in finding IBS-safe ways to help you gain weight
It can be tough with parents, especially if they're the cooks and the grocery buyers, but hopefully they can help you out.
Have you ever been tested for Celiac? Because that can cause a lot of weight loss in some people and make it very difficult to gain it back, and a lot of the symptoms are similar to IBS. Just a thought...