HI, I'm sorry you are feeling so sick. I know there is a lot of info on this website so try not to overwelm yourself and just take your time trying to learn it all. Heather's books are sooo helpful and easy to read you might want to start off by buying them or trying your library for her books.
It's okay to have sugar just don't eat the sugar free stuff because there's something in there that will bother your stomach. Also fat is okay to have and this diet is not fat free it's only low fat. Soy sauce is great over rice and I eat that all the time.
More importantly have you been diagnosed with IBS from your doctor? You need to make sure it's not something else. Are you eating enough and maybe that's why you are feeling weak? Although I have some of your symptoms I remember feeling very weak and tired when I was underweight. I used to skip meals all the time to avoid stomach attacks and that was just no good for me. I actually started feeling better once I started eating small meals more frequently. I do hope you feel better soon and take your time reading the info on this sight.