Do you still have the wireless enabled on your gateway? Was your computer the only thing connected to the gateway? Do you have any iDevices as all will sync to the cloud...I believe icloud backup and icloud photo sync are to different settings. IF you take alot of photos or movies with your devices they will automatically sync to the cloud unless you turn it off. Android devices will also sync photos to the cloud.
Also login to the router and check to make sure all the clients connected to the wireless are expected to be there. There is a simple WPS hack in the wild that can expose the WPS key. If someone has hacked you in this manner you can change your password and they get retrieve it again. Disable WPS on the router/gateway as an extra precaution.