Open Question: Why are my boobs so sore?

I noticed last week that my boobs were slightly tender. I thought maybe it was just how I slept or something, so I waited to see if it went away. It's been over a week now, and they are even more sore. My breasts usually get tender a week before I start my period. But, my period isn't due for another 2 weeks. (So, they are tender three weeks before my due period, because they started hurting last week.) Also, usually when they're sore due to an upcoming period, they usually are extremely sore, especially around the nipples. This time they are only sore if I touch them, and they're sore all around the sides. I am NOT pregnant. I am a virgin. What could this be? Could it be the way I sleep? I NEVER sleep on my stomach, ever. But, I have been the past few weeks because it's soooo hot in my house, and I've been desperate to find a comfy sleeping position, lol. Thanks for the answers! : )