I think it would be best for you to get a handle on your eating. Immodium isn't going to change your IBS. Feeling bad or guilty after eating like that isn't going to help much. Just move forward and quit beating yourself up about it. That gets you NOWHERE!
I have done the same thing myself. Since I don't suffer immediate consequences with my food choices, it is easy for me to go overboard at times. I don't get cramps or pain or even immediate D or C, but in the days to come I will feel it. You have to make a decision if you want health and to feel good, or to continue to eat the junk. If you want to continue to eat the junk, then you are wasting our time.
Just get on with it. Get on a good diet, exercise, use medication if necessary, and don't focus so much of your life on your stool. I guarantee you will feel better. Remember, what you FOCUS on expands. Focus on health and well-being instead of what's in the toilet.