If the membership records were expeditiously transfered, keep in mind also that the wards from which you were created no longer have the membership records for those who were in their former wards. I believe it would be appropriate for you to report information concerning those new members of your new unit. In other words, as you go into LCR as davesudweeks suggested, I would record attendance information at the very least for the names of the Young Men, Young Women, and Primary who are now in your ward and cannot be reported in their former ward(s). And if you are able to record priesthood and Relief Society attendance also, the record will be even more complete.
For sacrament meeting attendance, if your ward was created on the last Sunday of the month, the former ward should have reported everyone at that meeting and you will have zero for the quarter. I would check with the former ward clerk and find out how attendance is being recorded.
But if you had your own sacrament meeting on that day, or if the clerk of the ward from which you were divided did not record attendance for those who are now in your ward, I would not at all feel bad about reporting that number and treating that last Sunday as a combined meeting of the two wards (if it was) or as your first ward sacrament meeting. In either case, your "average" for the last month of the quarter will be the number for that single Sunday.
All of the attendance information has budget implications, so don't double dip on the sacrament meeting numbers, but make sure everyone is recorded. And make sure to record the attendance of members at their other meetings so that their attendance is recorded in whichever ward the membership record resides at the end of the quarter.