If you're not eating much, you are not going to go much. Input vs output. Also, you've been taking pain meds which can be constipating and that's probably contributing as well. A lot of pain meds are notorious for constipating people. The stress that you have been under with your mom being so ill, the holidays, your shoulder and pain meds all can contribute to an MS flare and/or the constipation you are experiencing. Try taking a stool softener or laxative or eat something with a lot of fiber in it (a bowl of oatmeal, lentils, etc. or a prune or two) and see if that helps. Keep track of what you are eating and how much as that could be a clue. If you are eating normally as you always have and your BMs were regular and all of a sudden you stop going, that issue needs to be addressed with your doctor.

Keep in mind we are MSers and constipation is pretty common for us. Keeping our bowels running smoothly can be tricky as is nerve related.