Swollen breast tissue in newborn

Niamh

So my daughter is 10 days old and the breast tissue on her chest is slightly swollen and red, more so on the left side. The midwife was out today and said if it gets worse we will have to take her to hospital. She said its caused by the passing of my hormones in the breast milk. I've never heard of this!! Anyone seen this before? Any idea how long it takes to resolve itself??