Hello chippy. I would endorse what Puppyluv says about seeing a medic about this problem. The fact that it comes when you sit in certain positions might indicate that you probably don't need to worry too much unless the pain becomes unbearable. Very often, this sort of pain comes when something like a nerve gets trapped and the particular position you get in will indicate to the medics what is amiss. However, there are several other causes for this type of pain, one of which is trapped wind, (or faeces which is often called a 'blockage') which can be very painful.
I have a colostomy which means there is more colon left to absorb the moisture from my output and, although I irrigate, there is sometimes a small residue of output that does not get expelled but lodges as little hard lumps in and round the stoma area, This can cause the type of pain you describe and it goes away the next time I irrigate.
When I first had my stoma, the nurse showed me how to digitally (finger) feel inside the stoma to see if there were any blockages. This has proved invaluable over the years as it allowes me to envisage what is going on just inside the stoma. Quite often it is something as simple as these small amounts of hard faeces. However, it has also meant that I can keep an eye on my parastomal hernia which completely closes the hole and wont allow wind or faeces to exit. This is a different problem altogether and I have resolved that by wearing a stoma plug, which keeps the hole open and lets the wind escape. I have also made a similar device which allows the faeces to escape but that is a different story.
Very often, I can more or less work out for myself what any problems are with my stoma and associated pains, However, once I think I've identified the problem, I usually share it with the stoma nurse, even if I have resolved it. This is because I believe that stoma nurses should have as much information as possible to help others in their work. They are alway grateful for my input and have been very helpful to me in supplying materials for my experimentation.
I hope you manage to resolve your problem soon and when you do, perhaps you could feedback to this group as to how it was resolved.
Best wishes
Bill