Hi Guys :)
I have to say I think that is the smallest 15hh horse I have ever seen! :P I hate it when they don't have the actual hight THEN the expected hight. Have you asked what her measurements are atm?
That said, I think what your noticing about her back is the fact that it's a tad long. Nothing terrible tho and like you say, she IS still filling out.
She's beautiful, tho she is a LITTLE chubby