Hi,
The simple answer to that is no.
We have no way of knowing how you use your iPhone and what details you keep on it.
iMessages logs in from iPhones, Macs and iPads and iPod Touches.
These are Device Specific using the Serial Number of the Device.
The device used for the Chinese messages should be listed on the Apple ID site once you have logged in.
That and the fact people are seeing it as another device shows that they have your Apple ID and Password at least.
If your iPad and iPhone Backups are in the Cloud then they may have more info.
If it is the same Apple ID you use in iCloud and you use that for Mail and Sync Contacts then they would have access to that.
If the Contacts had plain text info about your Bank details ...
Mail connected to IMAP servers will show them everything not just the "new" stuff. (Unless you purposely delete older stuff).
POP will depend on all the devices accessing it being set to Leave Messages on Server.
7:57 pm Sunday; July 31, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)