Registered my iPhone on my ex's Mac and now I have a ton of overages
Probably not, considering there is no way to access your plan or wireless service from a macbook, you shouldn't have to worry about that.

I would check the settings (view and adjust data use https://support.apple.com/en-us/HT201299) on your phone and ensure that someone isn't swiping it when you are not around and playing games (like some smart kids.)

If that doesn't fix the problem I would recommend calling your phone provider and investigating, someone else may have access to your plan via calling up your service provider and adding additional phones, or you may be billed on accident.