Hi Beatrice, it's probably because you're constipated. Even though you might be having a bowel movement every day you are probably not emptying your bowels properly, and that is when leakage occurs.
I would say you should see your MS nurse (if lucky enough to have one) or your GP. Both of them would have heard this problem many many times.
If you are not having a proper bowel movement every day then you should try a laxative. Again, GP could prescribe one or you could try an over-the-counter one from chemist.
Or the problem might be 'loose stool' in which case the GP will be able to prescibe something to harden it.
Also, if not already using it, try moist toilet paper, sold everywhere. Makes clean-up quick and easy.
Please don't ever be embarrassed to post anything on here! We are all very much in the same boat!
Hope this helps,