That would be my guess. Have you looked at your actual reports and seen if the account as updated? There is normally a last updated and if the reports you have don't show this then credit karma is good for seeing this information.
Your credit file does not update based on the day a payment posts. Its based on when your lender reports your balance to the credit bureau which is usually once a month shortly after your statement date for most lenders. So the date last reported per individual account may be different but will normally be the same time of the month each month and at that time if the changes to your credit file warrants a score change, it will change then (other than other items like aging of accounts). The change you're seeing may be related to something else. Also, paying all revolvers/CC to $0 brings about an AZ (all zero) penalty although that seems contrary to conventional wisdom, but it is the way the FICO algorithm works 
MyFico only provides the mm/yyyy of last reported but Credit Karma will provide the last "report balance as of" date. In my example this card reports the balance as of the 27th of each month. But it may take a few days for it to hit your report which means it's unlikely it will be updated on your credit report on the 27th but whatever the balance was on that date will be the amount reported. Each bureau has it's own timing. EX seems to provide the update sooner and this update hits my EX report usually by the 29th or 30th. EQ a few days later and for me TU is the last to update for all of my lenders.
Credit Karma
MyFico
