The only thing that I can think of that causes breast tenderness is pregnancy, but since you said you can't be pregnant it could just be hormone changes. You don't need to panic but I would have it checked out just for peace of mind. Just make an appointment with your OBGYN or primary care doc and ask them to take a look.
I have had many times when my breast were extremly sour. My period was not due and I wasn't pregnant. There have been times that the pain would last for 3 weeks or so.
I have Fibrocystic Breast Disease, the are cysts that develope in the breast that are non-cancerous. I have several of them in just my right side. This is the only known breast problem that I have and I have gone to a specialist numourous times. I would not say that you may have Fibrocystic especially if you feel no lumps. I would go and talk to your doctor. I wouldn't take the pain as a scare. Good luck