Lactose or similar should have been prescribed as with me when I was on a course of pain killers.
May I make a suggestion, gross though it is.
It turns out I have cancer of the oeseophagus. I haven't eaten anything solid for months, so nothing in, nothing out. I still feel the need to go though, and it has been annoying and painful to say the least.
I solved things by getting an enema bulb from Amazon for about ten quid. Best used when having a bath, suck water into the bulb, squirt water in anus, and then force back out as you would a normal motion. The water adds lubrication and pressure to eject what is in there.
Like I said, gross, but the relief I got was amazing.