Why does nursing hurt so bad??

THRUSH! I took Ella to the doctor yesterday to have her bum rash checked. Turns out it's not a bum rash but thrush and there were traces in her mouth as well. The pedi said if I had any sores on my boobs the thrush would be in my nipples as well and could cause severe pain. This is such a relief! We have met with several LC's and they said her latch was good so we were really at a loss as to why it was hurting sooo bad and not getting better. We started meds last night and the doc said within a couple of days we should  both be feeling better. yay!