Fast use of 8ta 2 gig pm allocation

With my previous ISP I had a one gig ADSL allocation pm which I seldom exceeded
I thereafter signed on for MTN's 3g offering of 2 gig pm where any unused excess was added to the following months which happened every month cos I seldom exceeded 1 gig usage pm
I now have a similar package with 8ta but the speed at which I use up the 2 gig is astonishing. I purchased 2 gig + 1 gig nightime usage on 1st April and my balance shows I've used over 1 gig of my daytime allocation in nine days despite that my usage habits have not changed over time and that I disconnect after using.
Please help. I'll go broke at this rate

Get NetLimiter and use that to monitor each application's Internet usage + your PC's total Internet usage.
This way you can ensure that the amount you're getting billed for is correct.

Lastly, do you have an antivirus installed on your PC, or are you using OSX/Linux? If you're using Windows and you don't have an antivirus application installed, I would suggest that you get Microsoft Security Essentials, but this would be like a once off download + update of about 150-200MB. It would try to perform daily updates, but it shouldn't be that big: perhaps 20MB-30MB a month or so.

So far my svchost.exe (which includes Microsoft Security Essential & Windows updates) has used 120MB this year on my gaming PC.