Re: I don't know what to do
This is a handy reference on diaper out out in the older baby: http://kellymom.com/bf/got-milk/supp...ughmilk-older/. According to kellymom, you're looking for 4-5 sopping diapers per day, and in order to feel what a sufficiently wet diaper feels like, you should "pour 4-6 tablespoons (60-90 mL) of water into a clean diaper (if baby wets more often, then the amount of urine per diaper may be less). Diapers may be wetter in the morning, especially with older babies. Urine should be pale and mild smelling."
One thing that often happens with older babies is that they discover how to hold their pee for longer, in order to avoid the discomfort of sitting in a wet diaper. That can make it look like there's been a dramatic drop in diaper output.
As long as you are continuing to nurse on demand, I don't think there's any reason to supplement. Just keep an eye on the diapers.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"