RETTWINS

So starting from a dead stop I put all my weight into the pedals to get out of the way of a car. Crunch!!! Awful noise. Kind of felt like it came from the BB. Went to skip the bike a second later and Crunch. Same sound. Rode for about another 20 minutes with no problems. This morning I get up and ride for a good 20 minutes with no issues and then crunch again when I went to skid. A few minutes later I stepped down on the pedal and it felt like it slipped. But then no trouble again in my 30 minute ride home. I'm thinking maybe I pulled the rear cog loose (or stripped it??). How do I check? Could it be something in the BB? I noticed my rear wheel was a little off center. Could I have pulled the whole wheel in the dropouts? Any suggestions on how to figure this out?