What Causes Tenderness/soreness In Nipples?

Hi,

I noticed yesterday while out on a brisk walk that my right nipple was really tender..it stayed like that throughout the day and it wasn't until I got into the shower when I realized I had a sore on my nipple..It hurt when hot water hurt it and with wearing bras, but it's a little better now, kinda scabby. any ideas what this is and why it was created? Thanks