Why does my body think it has to go so badly but then nothing happens? Is it just there and stuck?

I have IBs-D. I’ve had enough times when I think I have to go and sure enough I do that I never question the feeling of thinking something’s there to come out. But often times it becomes more of a safety measure like better safe than sorry and when I sit on the toilet nothing comes out. This is not the worst thing but is very frustrating when it really feels there’s something there (and I don’t think this is constipation) and nothing comes. What’s going on with my system in those situations?