iOS 8.4 Update - data usage
I do not know if I had this problem because I have unlimited data, but my friend did. Notably, she also has a different carrier (Verizon) with limited data and when she noticed the high data usage she contacted Apple. They acknowledged that with some carriers this is a bug in the newest version of the iOS. The fix they had her implement, until the next version of the iOS comes out, was to revert to the prior version. Since the update had completed, she was forced to do this through a full reset. Since the phone came with an earlier iOS version, this is what it reverted back to and the issue was solved. Again, I am only reporting this as an observer and have not fixed/experienced personally, but thought I would offer this in the event it helps.