I wouldnt assume to know weather it was the bottle of water or not did the damage
But having suffered from hippains since last easter when i started training for my own camino i can not help myself stressing that any unusual pain is not to be taken lightly. I know this because i didnt listen to what my body was telling me and am now on the 11th doctor/therapist (and this one actually have an idea for a solution) and have had days/weeks with severe pain during this last year. (Initially i blamed my hipbelt and my shoes, but who knows)
you make no mention on the specifics, but leg and hip-pains can actually stem from the back and you do mention having had issues with your back (but may hurt too).
If I were you I would take it real easy if I were you and maybe see a physiotherapist, also as I have now learned that hippains may stem from the iskias nerve.
Not trying to scare you, if is gone after a few restdays and never come back you are probably fine, just saying that i know from bitter experience how much one can hurt oneself setting off on a long walk injured...
I also know however, (if it is any consolation) that one learns to walk past the pain and that one can probably walk slower and shorter stages than i did, which is what i plan to do this year
btw bad posture can often largely be fixed at the gym