Broadband usage

[#1]

Today I've received the following message from PlusNet:
'Please be aware you have exceeded your included monthly data transfer of
1GB  - there is no need to worry.
An extra 1GB of data transfer has been automatically added to your account
for this month at a charge of £1.00.
If you use up this extra 1GB before the end of the month, another 1GB will
be added at the same cost of £1.00. And so on, until the end of the month.'
This is mystery to me as I'm a light user with around 500MB of usage each month on average, and have no idea how I could have exceeded 1GB in the 3 weeks since 7 October. The PlusNet message ends:
'Monitor your monthly usage -
http://portal.plus.net/view_my_broadband_usage/index.php'
Yet whenever I attempt to do this I get the message:
'You don't have any usage data to show right now. This might be because your account is new, or your usage has been reset for a new billing month. It can take up to 2 hours for your usage to show up - please check again later.'
What can I do to try to pinpoint the cause of this sudden jump in usage please?

[#2]

Hi there,
I'm going to send you a PM as I don't want the entire world knowing your usage

[#3]

In addition to your usage which James is PMing to you, the error message is being received because you are still connected via RIN (plusdsl2.net), you will need to switch back to username@plusdsl.net in order for the usage to show up on our account.

[#4]

I'm much obliged for both pieces of information.