That is a huge jump.  Have you installed any new apps recently that could be sucking data?  Or perhaps using an app that you never used much before.

I have been experimenting with an app called My Data Manager that provides an analysis of data and Wifi usuage by app by day.  If you have an android phone, it might be worth downloading it if you also have android.

Most phones also have an app included in the provided apps that shows data usuage for the billing cycle.  Have a look and see what apps are using the data.  That would be a good starting point.