Paid on 12/14/15; Upgraded on 12/19/15 double billed?

I paid for my monthly service on the $25 plan on 12/14/15.  I paid $35 at the time.  Then on 12/19/15 I went to the ATT store and upgraded to an HTC Desire 626 phone using the $45 monthly plan.  I paid an additional $35 dollars at the store as instructed by the tech there.  Am I wrong to think I am being double billed and not prorated for the amount already paid for the month?