D.W.
Our pediatrician had us start rice cereal and level 1 jarred foods at 4 months for both kids. Ours weren't so natural at it, and most of the food was wasted, but they have to learn, and it sounds like your daughter is a natural.
I always made the rice cereal with pumped breast milk so he continued to get the nutrients. I had to stop nursing our daughter to start chemo when she was 11 weeks old, so I'd make it with formula. We'd also put some of the level 1 foods in there to introduce different flavors.
Your breast milk supply is probably fine. She's just developing the need for other foods. Your milk should and will continue to be a primary source of food, but she's giving you clues that she's ready to need other nutrients for sustenance.
Stick with the suggestions people have provided (cereal, veggies, pears, applesauce).