After 5 days you should be feeling some relief on the back ache - so if that is still not better by day 7 I would put another call into your OB and ask there. I don't recall pg related back ache's in the first trimester.
You will be peeing alot for the next several months
Definitely up your water intake and add in cranberry juice if you can, not the blend kind, so I usually dilute it.
Also switch to cotton undies if you haven't already. It really does make a difference pg or not! I wore boy short style underware during pg and it was very comfortable.
Hope you feel better soon!!!