Welcome to the community!
Where does it show your data is maxed out? On the Fido app? Did you try to check online maybe there is a glitch with the app and you can try clearing the app cache and see if it helps?
You should also note it takes around 3-4 days before the invoice is produced so if you got your invoice on the 1st it means your cycle reset a few days earlier and depending on how much data you have and what you used it for it is possible you used it up already.