All that could go wrong just about did

Due to all the problems it turned into a out and back to Sealer bay (I slipped and went down on wet boardwalk soreknee/good wack to back of head hit by pack/creek at high tide daughter fell with pack drenching her bedding/bird stole food while in toilet - then possum broke into verstibule overnight and took rest of food so we had really none left - etc)
We used hire backpacks - and although it felt like the weight was on my hips my shoulders across the back of my neck are so sore I cant touch them.
Just wondering - is that just normal and I will develop more strength there? (ie suck it up and practise more) or is it likely the pack didn't work well for me ?- it was fitted by the suppliers Calves etc all a little sore and a minor hot spot or two on the feet but nothing at all of note - just the shoulders (not the back) any ideas please? Daughter also a bit sore at exact same location.
I don't want t be a princess - but don't want to struggle through and find out it was a easy fix
