Prepaid credit disappeared

Hi, something strange is happening with my credit balance. 

I’m using German prepaid number and prepaid credit with mobile data. Last time I refilled my balance on the November 14th with 15e. And now, four weeks after that my credit balance is 0.08e. The problem is that I only spent 8.5e, and the the other credit vanished. I downloaded the PDF which confirms my costs, but could I send a complain?

And this is not the first time that something like this is happening...