Happened to me too. Hello globe?!!?!?
It happens all the time. Even though you didn't open your mobile data.. the load keeps decreasing when your phone is connected to the internet through wifi... its eating the load
What I did is to enable SURFALERT by sending "SURFALERT ON"
then I sent it to 8080. This way, if the data limit has exceeded your
plan, the automatic surfing will not eat up your regular load.
So far this works for me.