My second daughter put on 100g between 3 and 6 months. Nothing wrong with her at all, it was just her changing from the fat bubba I grew (10 pound) to the body size she is supposed to have. When I look at the charts she was much bigger than the other two kids at the start, but by 6 months she was tracking along pretty similar to her siblings and continues to do so.

ETA, if people aren't being supportive of breastfeeding, just don't mention her weight gain.