How can I find why I'm using so much data on mobile network? According to my provider, I've used 6.8MB of data within 3 days. 

The thing is, I don't know how. I've installed DroidWall and WiSync and only used Waze (a navigation app). I installed Traffic Monitor which shows Waze using less than 800KB and other apps even less (some of them used traffic before I setup DroidWall)

Any clues as to what is using 6MB??
My Data Manager is a free app available in the android market which will aid you in catering your needs. It will give you a detailed statistics of which app/service used how much data.
Edit : did not notice this was answered in the comments already.