Frustrated (impatient?) eater

VineyardMel

DD has been getting really frustrated while nursing, crying and pulling off at the end and continuing to act hungry. Sometimes after I burp her and we move on to other things she's just fine, but other times she's still fussing but won't take the breast. This happened today she was acting so hungry so I gave her some milk I'd pumped earlier and she gobbled down 3.5 oz. She doesn't get many bottles but I've seen her stop when she's full a few times so I don't think she ate it only because it was there (and I started with 2oz and she wanted more).

She's got plenty of wet diapers and seems to be gaining and thriving but more and more she's acting like she's not satisfied. Today I checked and could hand express some milk, so I think there's something in there, but maybe it's not flowing fast enough? If it is just a patience issue, anyone have tips on how to get her to keep eating when she wants it to come faster? I think it must either be too slow or my supply is slowing down and I want to try to fix it either way.