Sometimes I think those charts were created just to torture us moms. My LO was "off the bottom" in everything but length for some time, now he's up above average in everything.
What sort of symptoms other than the weight thing is she having? Is she having a lot of gas or diarrhoea? I probably missed posts as to some of this. If she is eating healthy and acting healthy my hope would be that she's just going through a phase and will hit a spurt soon.
Honestly, you can't blame yourself -- we look at our LOs and see nothing but beauty... you're the best of moms, even after all you've been through.
I'm sure she will be fine, but please keep us posted as to your doctor visit.