Hi K.,
Don't worry, I had the same issue with my first son. I know it sounds strange, but once you read on, you'll feel so much better.
I am assuming that you still breast feed your daughter? If that is in fact the case, what is happening is that breast tissue is forming because your daughter is ingesting the hormones that are produced for your body.
When that happened with my son...and I found out the reason, I cut back on the breast feeding and weened him off completely. As the breast milk slowed down the left breast he had slowly went down and eventually disappeared.
However, if you are not still breast feeding, then I have to admit, I am not sure why your daughter would be experiencing this.
Here for you,
L.