My DD was a very active baby and actually lost weight between 6-9 months, and then again between 9-12 months. I was a bit worried, but she also grew 4" in that time. She was (and still is) perfectly healthy. It's normal for most babies to really slow down on gaining around that age, or even plateau. As long as she's happy and hitting milestones and eating well, it's probably nothing to worry about.