Girls, how do I know if my milk supply has decreased or is established…

Girls, how do I know if my milk supply has decreased or is established? It suddenly became low. For two weeks now, I've been trying to express, and I'm using a breast pump, but it doesn't seem to be working. I used to have firm breasts, milk would flow freely, and I gained weight well, I could express a lot of milk. Now my breasts are always soft, they don't fill up, nothing comes out when I try to express, at most 20-30 ml. We will only find out through a week's worth of supplementation that we need to see a doctor. Should I supplement with formula right now, or is this just an established lactation and everything