Just had someting similar happen with a steam beer using dry yeast....brewed it up Friday night, Saturday the airlock was bubbling away, Sunday the activity was next to nothing. My gravity readings tell me it is pretty much fermented out. My gut, and my friends here at HBT tell me everything is normal. Racked it to a secondary so now I'm just letting it mellow a little more before bottling.
I'm guessing you're in pretty good shape. Let it go for a few more days and check it. In the meantime, RDWHAHB!