Have you dropped your chain recently? I broke a rear spoke recently and when i had it replaced- they pointed out to me that 4 OTHER spokes were slightly bent and gouged as well- right near the hub. Apparently when I dropped my chain once going up a hill and shifting badly, the chain dropped down between spokes and gears and damaged several spokes at once before I was able to stop and put the chain back. At the time I didn't realize I had damaged any spokes, but one broke a week later and that's when they pointed out the other gouged ones. The gouges were not that obvious unless you looked for them, but the spokes would likely have broken one at a time if we had not gone ahead and replaced all 5. Checks for bent or nicked spokes near the hub, especially if you have dropped your chain at some point.