Cook your own meals. Specifically, cook your meals like you would eat at a Southern meat-and-three diner, or perhaps a meat-and-two, at which you would have a meat dish with two/three vegetable side dishes plus bread as your main meal of the day. The high vegetable content should calm down your angry colon and result in healthier digestion. Source; experience
The point is to start eating like great-grandpa did back in pioneer days, eat healthy and eat home-cooked.
Oh, by the way, that's a recommendation for long-term problems. For short term, I don't know, take a laxative? If it's powerful you'll feel like you're dying while you shit, but it will do the job. Avoid getting constipated (i.e. eat veggies and fruit, not just meat) in the future. I had bad poops, then I started cooking instead of eating out all the time, and it mostly went away.