Does she have plenty of wet and dirty diapers? Does it seem like she is getting enough to eat? You could do weighted feeds just to ease your mind about it.
My boy is 1 year old on Sunday and he had problems with gaining weight even after we started supplementing with formula at 5.5 months and even after he stopped latching just a week before 7 months and I started pumping, I knew exactly how much he is eating and he also had lots of solids and he was still struggling, was born with 3420 gramm, was at 5-15% for weight all along, but now at his 1 year appointment yesterday he was 9540 gramm which is 50%, so despite of all our struggles he finishes his first year with a perfectly good weight