hi weeb. sorry this is happening to you.
for starters i think you need a urology consult, maybe even an MS urologist.
call your dr and tell them you want it.

i'm on detrol LA and i take it at night. odd i know. but what was happening to me was i was peeing in the bed at night and also when i got up to go i couldn't make it to the BR. switching to taking it at night from in the am really solved the problem. the urologist's nurse suggested it.

i have spinal cord lesions and they think that's why the signals aren't getting thru.

for me, i drink about 2 quarts of water a day. i stop drinking at 7pm.

i also take metamucil at night and good luck with it.
but sometimes trying another product will work better for some people.
you might also try a stool softner. NOT A LAXATIVE, but a stool softner.
ask your pharmacist. you can get the store product that has the same ingredient for much less $$$.

i think you should press your dr to address this issue. if something's not working make another appt. you might even need a GI consult altho most internists should be able to help you. there are also prescription products to help with constipation. eating fiber would be my first suggestion, and fluids like you said.

i hope you get a handle on this soon.