You could well be down in traffic, and you'd not be the only one. It happens.
I had a huge downturn in traffic for all of September and October - I mean not just a little bit down, way a huge percentage in the negative.
Around late October (assuming I'm remembering my months right) I recovered and then some.
Sometimes Google puts subdomains into "the sandbox" and sometimes Google is testing their search results with or without any of us in them.