I pretty well have suffered the same infliction, it is a well know fact that the side effects of narcotics is and can cause chronic contipation, I have also had the problem of several lumps in the areas you describe, you woud especially notice them, because you are not unable to relieve your bowel fully, what I do believe  happens is when you apply to much pushing or bearing down, the hard impacted stool that your are unable to pass ,has no room to manouver it will tend to too push itself outward any way it can, untill as you are doing removing it your self personally, in doing what you are being put in a position to do to relieve the bowel, you could actually cause your self rectal tears, which could cause black stool, I always understood that black stool usually was caused possibly by hidden bleeding or ulcers ect come from the upper end of the large intestine.

           We you are forced to do this painful act, to get relief, one thing from my own experience is do not apply a forceful bearing down to try to empty the bowel, my you have to I assume you use protective gloves, which I apply petroleum jelly, coating a your index finger liberally, so when you have to do what you have too, the gently coat the jelly as best you can around the particular stool that you need to extract, yes were tallking the stool that is stuck in the side walls of your rectal arena, try to do your best to reach as far as you can to scoop our the rectal area, but take your time, I am no means a doctor but I do know exactly what you are described, I have on occasions for almost 2 years now, have had to due that painful ritual. I do not know if you have health ins, and cannot afford to see a doctor, to prescribe something for you, to help eleviate your problem, if you do not, try to keep your fluids up, try to put more roughage in your diet, fresh veggies, flax seed is really cheap really no taste to it , you could put it in every dish about a teaspoon a day, to help keep the stool soft, or buy a stool softner, I have gone as far as giving my self a baruim, you can get a kit at most drugstore, it is a bag that holds a very small amount of non scented dish soap will do, usually instruction do come with the kit. the kit helps  you flush your lower rectum to help removed impacted stool.The best place for that is in your bathroom, cause when it decides to do its thing, being in the washroom is real handy. But I really recommend to see your doctor to help choose the right procedure to relieve you of this problem you are suffering , the more you eat and are not able to relieve the impacted bowl, the more you will get alot more bulging of the pelvic and belly button are, plus no doubt added abdominal pain, vomiting is a possible too cause your poor colon can only hold so much.

               So I hope I did not steer you wrong on the basic idea what is occurring because as I said been doing on and for over 2 years as I to TAKE, very powerfull narcotics...good luck