No one here can view your bill. If you download the PDF version from My Telus, all the details should be described there. Is it possible that your last payment was received late?
Also see this post for a description of a similar issue.
I just received a letter saying that my payment is past due but I actually have a credit in the account, which means Telus owes me $200+ and the bill actually said no balance owed, I suspect the program that generates these past due notifications has a bug...very disappointed (since I am a programmer and know that this is so fundamental and totally unacceptable that the program cannot differentiate between a credit balance and balance owed.....👎
In every billing/accounting system there is a cutoff date for reporting and generating correspondence. If events happen after the cutoff they will not be reflected in those reports or correspondence. You say you received a letter, I assume you mean physical mail via Canada Post. If that's the case you also need to consider the time it takes to generate that letter and have it delivered to you. Just because you received it today doesn't mean the balance on your account today will be reflected in the letter.
What date did you make your last payment and what date is on the letter?