Tech came by... apparently my plan had been reduced to 6 Mbps 'U-verse' this month. Checking my account emails there was no notification of a service downgrade. I'm on autopay, and I did receive an email last month saying the monthly bill would increase by $5—but no indication anything was required on my end.
I was last billed on 6/17, and my next bill is due 7/17. The service tech said he's seen this happen before, and it is likely related to AT&T's backend billing system being a mashup between four different systems across three continents.