You know, I've typed several responses to this but have deleted every one. Is this (the fact that the pain may be psychosomatic in origin) something that has been suggested to you, or are you trying to make sense of the symptoms? I ask because I'm afraid to say that doctors are oh so inclined to pigeon hole pain they can't explain, or do anything about as being pyschological in nature, since it lets them off the hook. If I sound slightly bitter about this, it is because I have dealt with this issue in my own family, only to find out that the pain was very real. Actually to be fair, pain that is pyschosomatic is real - what I mean to say is that in our case there was an actual organic, treatable cause for it.
I assume that the previous cause of the pain has been ruled out? Properly, I mean, not just someone saying "I don't think it's that"? Something that I think does/can happen when someone has dementia is response to pain which is akin to what you would see in a yound child - pain is all encompassing and then, suddenly, it isn't. I think that much of the way an adult deals with pain is based on personal experience and social conditioning, and of course, when somone has dementia, all that is lost. When I, for example, stub my toe, it might bring tears to my eyes, and foul language to my lips, but I know that it's transitory - the pain will dissipate, but for a child or an adult with dementia, that knowledge isn't necessarily there. Thus any pain is an entirely unexpected event which has no rhyme or reason. No wonder it reduces someone to terrified tears.
That is quite apart from the fact that all pain messages are processed in the brain. When there is an organic disease of the brain, I think it is entirely possible that the brain receives mixed messages from the nerves in the body - a message is sent that shouldn't be interpreted as pain but is misread. Also, I would also think it is entirely possible the the gate mechanism (which, simplistically, is where the brain receives as message of pain, and then effectively says , OK, I've heard this, I don't need to keep hearing it) becomes faulty. That failure is the origin of many neurgogenic pain syndromes.
None of which help you or your mother. What does her doctor say? If it's possibly related to anxiety (and while a change in enviornement might produce mild anxiety in a well person, one can see that such a change might produce truly gut wrenching anxiety in your mother) could not an anti-anxiety medication be given a trial? Alternatively, if the pain is of short duration (and assuming she then does not remember it) I think you'll have to experiment with various comforting techniques, as you would with a child. A stuffed animal to hold on to, reassurance that this pain will pass, simple human contact can all act as distractions until the pain has passed.
You will note that from what I've said that I think that it is unlikely that the pain is truly pychosomatic in origin. I think it is more likely that there is some level of discomfort which is magnified in her mind, and all you can really do is provide the reassurance that she needs that this too shall pass. Actually, even if it was psychosomatic, I doubt there is anything that could be done medically anyway, except possibly anti-anxiety meds. She could hardly cooperate with the normal "talk therapy" approach to such illnessess.
best wishes
Edited to add: I note you mention senna. Now this is very much a personal issue, but I find that I get severe, and I mean severe (as in stabbed in the stomach), cramping when I take senna. You might want to investigate an alternative.