What could cause this pain in my left breast?

Have you felt your breast where the pain is? It could be a cyst. It just feels sensitive and is a little harder than the rest of your breast tissue. It could also just be the way you slept-- like if you slept on your stomach. I'd wait a few days to see if it goes away.. if it doesn't then you should see your doctor because it's the only way of knowing for sure.