Hi Emily & welcome to the BF forum
I think Suzanne may be right on the money.....the "mustard seeds" are actually undigested little fat globules and do show that the baby is getting lots of the richer hindmilk that the breast produces later in the feed. Do you feed on both sides right now? How long on each? Does baby come off once the intial, fast letdown slows or does she stay on for a while with the longer, drawn-out sucking/swallowing? Breast compressions could help keep her interested in staying longer on the first breast and getting more of the hindmilk.....
If you are feeding only on one side, block feeding may be the answer....only feeding off one breast for a set period of time (3-4 hrs) regardless of how often she nurses....so that she'll get more of the richer hind milk from the breast before switching to the other's intial "fast-flow" of foremilk.....
Don't be deceived by the weight gain....foremilk bingers can put on a TON of weight very quickly too......another clue to watch for could be a greenish tinge to the poop.....