I get panic attacks, too. I have been on Librax for several years. I take it before meals and it relaxes me and relaxes my tummy, too. I was taking 3 a day and then 2 a day. Now I try to take only 1 a day but on stressful days I go back to 2. I think it has really helped me.
Usually after my panic attack I get very nauseated. Sometimes get "D", too. Fortunately, my attacks have become less severe lately.
I don't take fiber, either. My IBS is mild and I don't have a lot of the severe pain that others describe. SFS's don't agree with me. As long as I watch my triggers and that helps, I'm going to stay away from them. A lot of people have been helped by SFS's, though. So, you need to do further research before you decide.