Welcome to the community!
Sorry to hear you've incurred an unexpected long-distance charge. I understand you were meant to have a pre-paid plan, however, if you are receiving bills, that would seem to suggest your plan is post-paid. You should note this forum is community-driven and not intended as a venue for customer services. We would not have access to customer account information. You would need to contact customer service to verify which type of plan you actually have. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
.... And I didn't leave Canada recently. ...
To clarify, long-distance charges incur when you dial numbers outside of your calling area. If your plan includes Canada-wide minutes, that would mean dialling non-Canadian phone numbers.
Those charges would be different from using your phone to make calls (or use other Fido services) while you are travelling outside of Canada. In that case, the charges would be considered roaming (certain situations might incur both roaming and long-distance charges).
In addition, it should be noted that if there are sufficient funds in your account, it is possible to make long-distance calls and/or use your Fido services abroad with pre-paid plans.
Hope this helps 😀