If pain started immediately after surgery I would say something might have gone wrong. If it didn't start for months, year or so it could be scar tissue which can cause all kinds of problems. They say scar tissue doesn't "hurt" but it depends on where it's at, how much you have and what it's pushing onto. The bad thing is every time you have surgery it just causes more scar tissue.
I am like you as far as constipation, I take a med called symproic which is basically for people that are on narcotics to help poop. There are several meds out there that are for this and can be a trial and error situation to find what is right for you.
I am not a doctor I am talking from experience and so many years of going through this.
Have you gone to a pain clinic? That can make a huge difference if you find the right one. Some have had surgeries or complications from, etc, so they understand. These days they are about only place that will prescribe pain meds and anything else that will help.
I was told I needed to have my colon resection. That lining of my stomach were separating to it was all in my head. So don't get frustrated by being told untruth statements, keep looking for doctor that will listen to you. Write down all your questions and take with you. If a doctor is not taking the time you need with them, go elsewhere.
I hope I helped a little bit, message anytime.
GOOD LUCK