If you had B Pay sent before cut off time on that Friday, then your payment would be sent for clearing that night, arriving in your Citibank credit card Sat morning.
If you did your B Pay on that Friday after cut off time e.g. evening, then your B Pay had missing Fri night / Sat morning clearning, so it was queued for the next clearning. There is no clearing on Sat night, no clearing on Sun night, so the next clearing is Monday night / Tuesday morning. Translates to your payment being 3 days late.
Why did you leave it the last minute? This is not the fault of Citibank, but your own.