What Causes Soreness In Nipples After Missing Depo Shot?

Hi I've only had one depo injection on June 10 2010 I was suppose to go back for my second shot on September 9 2010 but I didn't. Three days ago I noticed that my nipples started to feel sore and they're still sore as of today. I took a hot late afternoon today but it came back negative. Can you please help?