Not feeling full

I'm 4 weeks PP now and I'm not feeling engorged, thank goodness, but I almost have a hard time telling if I'm filling up. She's been attached to the boob non-stop, she's gaining weight, but when I pump it varies between 1oz to 10oz. The variation has me scared as well as her cluster feeding.

I do feel when "letdown" happens occasionally but I just feel like my boobs are drained. Once the initial engorgement is done is there supposed to be a way to tell how much is being produced?