Last month I got my throttling warning at 11 gigs. This month I am at 3.8 and I got the warning. Also, last month I started using "Tether" app that was available on the iTunes store for a day before it was removed. The day after I used it I got the warning last month. I used it again Saturday for the first time this billing cycle and Sunday I got the warning again. I'm convinced this is not a coincidence.
You probably went from < 2GB to 11GB while you were tethering. If you're streaming any media, downloading, or even playing online games over tether, it will chew through a lot of data quickly.