It really sounds like an "MS thing". I haven't had this particular symptom (yet) but I have heard about it and MS does effect the nerves that regulate that bodily function.
Do you have a Neuro that you see for MS? I would let them know and see what they suggest. A GP will probably suggest the typical laxatives but if it's MS related a Neuro will probably know more about what to do.
I'm sorry you're having this problem but I am glad to see you posting again!