Updated to iOS 8.3 on Sunday.  Also signed up for the iCloud Photo Library.  Set the iCloud to "Optimize iPhone Storage," which keeps low-res versions of the photos on the phone in lieu of the originals.  Everything synced over WiFi and all seemed fine.

Now, whenever my phone is not connected to WiFi (which is essentially during my commute to and from work), it is gobbling up data.  1.14 GB yesterday and 541 MB for the first half of today.  I don't stream music or use the phone while driving, so it's got to be something happening in the background.  And when I jump back on WiFi, the data usage returns to normal. 

Is this a bug in the new iOS or the iCloud Photo Library?  Anyone else having this problem?