Log into your self serve account using secret/incognito mode. Go to the payments page. Refresh the page using the little round arrow at the bottom right. View your payment history. What charges are showing in your payment history for December? Do you have a corresponding balance in your available funds? Give us a screenshot of your payment history if you like?
@andrewmcinnis check your Payment history on My Account (using Incognito mode) and it will show you what and how much you got charged for
and if you use "change plan now", then it is normal that PM will charge you new plan cost immediately and the "old" plan's remaining days would be wasted