When did you upgrade your phone? I suggest you double check the 1st bill that you paid the tab and this bill to make sure it was 24 month period.

Also you can share a screenshot of your Tab history page here ( without personal info). And tell us what is your billing cycle. Then  we should be able to tell you what is going on.

It was a 24 month period paying it off each month but my tab charge now says $0 after paying my bill last month, yet I just paid another $10 tab this month...

Typically down to 0 means a final payment is upcoming on the bill. You could count back if it adds up to 24.