Downloads between midnight and 8am don't count towards your P2P/Usenet/FTP traffic allowance so you can continue to download up to 100GB (which also includes any peak time usage) during that period.
The level 1 threashold is 20GB during peak time (8am -> midnight) and the level 2 threashold is 25GB so it sounds like you exceeded the level 2 threashold by Monday.
I suspect the reason you did not get put on level 2 on monday is due to the way the manual traffic management is applied - i.e. they can only set the levels in sequence and probably can only change the level once per day. Others have reported similar level change emails appearing on consecutive days. This will probably change once the fully automated system is in place.