Slowed weight gain, supplementing?

At my daughter's 6 month wellness visit yesterday we were told her weight gain has slowed. My first daughter was on the small side as well, always 5-10 percentile. My second daughter, however, isn't even charting now. She's dropped percentiles and it's made her pediatrician and I worried.

I breastfeed on demand. Every two hours during the day and night. I've been trying to make her nurse even more during the day but she will push off and get frustrated with me. She's just not hungry!

Does anyone have any suggestions? I'm seeing an LC Friday but it just seems so far away.