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Did your actual plan change or did just the monthly amount increase? Is it possible you had a promotion that has now ended? Also, I know a few plans did have a price increase in the last couple of months in any event Fido would have notified you on your invoice of any upcoming changes. If this is the case you can check the last 3-4 invoices before the price increase and you would see the notification and the reason.
If your plan changed entirely and you did not authorize it then you would need to contact customer service directly and they can look into your account a better help you.