We are seeing the same issue over the last 2 billing cycle. It is very strange since we've had nothing but rain and we've been stuck at home connected to wireless. Yet 2 months in a row we've gone over data. The only option we have is to go unlimited.
I suspect suspect this is the same issue seen in the thread a year ago when people saw unexplained data usage increases.
I suspect it is the same issue, too, an increase in data usage due to changed settings on an installed app due to an app update.
Have you checked your device's onboard data counter which indicates how much data each app is using? Have you compared the current usage against past usage? Have you gone in to the settings of the apps using the most data to see if the settings are such that it can use data without your knowledge? Have you seen data usage by apps you never or rarely use?
The only reason unlimited would be the only option is if you are unwilling to do a little investigating on which apps are using the most data and then unwilling to change the settings on those apps to limit their usage. I have had this happen to me in the past and simply changing the settings on installed apps have put my reported data usage right back to what it was in the past.