I just brewed my 2nd batch of beer. The fermentation to about 3 days to start doing anything, and I was about to pitch more yeast when the airlock started bubbling and I got some foam on top. Today, about 10 days later, I checked on it and it is bubbling vigorously (not just the airlock, but the beer itself) and has gone from semi clear to opaque. The head on the beer has mostly subsided. What is happening?