MS-Stool & Urine Retention

I have MS. I have been retaining my urine & stool.
I have very bad back pain right at the bottom of my spine. I believe at this point it is due to not getting out all of my stool.  I take Mirlax to help with constipation. About once a week I have a decent bowel movement, but can't quite get it all out. This causes me to leek stool through out the next couple of days when I bend over or sometimes just when I walk up stairs.  
When I reported this to my Neurologist  he sent me to a Urologist, who now has me taking 0.8 mg of Flomax which has helped me to empty out my bladder.
I plan on seeing a GI doctor next.  
Any input or suggestions to help would be really appreciated.