Data just vanished

I have a contract with you for 1G night and 1 G day so last night I checked my night data balance and it was 999 MB and I decided to download a file of about 385MB and my bill was at R310 or so before download after the download which was supposed to use the night data my bill is at R1089 , mind you the download I did after 12 AM . I don't know what happened by this can't be. Somebody explain to me what happened because I can't go through this Again