Data vanishes

Every single month without fail data just vanishes.

I have downloaded wifi data monitoring apps, I've kept track on data usage myself.

I've taken downloads on wifi off, activate it when I'm on other wifi sources, yet rediculous amounts of data are being used. I can purchase 1 gig of data for my cellphone and that will last me 7 days easily. But the second it comes to my wifi ****le - eg. Wifi data replenished on the 1st may, it is now the 8th and 10 gigs have been used.

This is becoming rediculous. It is impossible, no downloads, no streaming, no updates yet 10 gigs gone in 8 days.