Ok, I'm not a brewing noob, I've done dozens of batches of beer, from extract to all grain and a few batches of mead. The other day I started my first cider. Pretty standard dry cider recipe with WL English cider yeast. Fermentation started quickly and 2 days later I had a nice krausen. Today I looked and there are what looks like chunks of yeast swirling around like something is stirring the cider. Considerable convection is coming up from bottom to top and back and swirling around. If I didn't know better I'd swear someone had just shook my carboy up but it just keeps going. What the heck is up with that? Is that normal?