VPN over cell network has made 3gb of traffic from my data plan vanish

Hello everyone.

Recently i've started to use VPN on my phone. Everything was just fine until today: i've got an SMS from O2 saying that i've exceeded my monthly traffic (6GB). My phone, though, shows the same 3.3gb of traffic that it showed a week ago (all this week i've been using my 3g connection only for text and occasional browsing). Could it be that the VPN flushed 3gb of my traffic in a week? How is this possible? (because i don't see any other reason so much data could just disappear without me noticing it)

I use Android and VPN via L2TP