Same here. I tried to turn it off, but it was too late and it wouldn't stop. Oh well. The billing cycle is almost over.I did over 2 GB in about 15 min on LTE downloading an iTunes playlist from iCloud.
There's a bug in there in that it continued to do so even after I had disabled cellular data for iTunes. Probably because I started the download before I disabled it.
I'm sure there was something you did to trigger the usage, perhaps iCloud related like me?