I second the WHO charts. Both my kids have declined on the conventional charts. First didn't double her birthweight til 7 months, second has just gone 9 months and still hasn't. But on the WHO charts, designed for normal breastfed babies, they're both pretty much okay. If you're worried about supply, then feed feed feed. Keep offering it, especially through the night, if she asks.

It's worth noting that around this time your supply may go through a "settling down" phase, where it's so perfectly attuned to your baby's needs that you breasts feel softer and it's hard to express. Fear not, the milk is still there. But your production has become much more efficient.