I have made one payment to SE and that was 7.69 pounds. In my account I had 20 pounds. That was the available balance. My current balance now is 3 point-something-pounds. I went to my bank to check what was going on and the staff stated what the title is stating.
Have this has happened to anyone else?
Maybe you hired an extra retainer ?
If my memory serves me well, it charges you with 1€/$/£ when you add a new CC, and you get it back after some days. Maybe that's what happened, with a weird error that charged you multiple times (assuming that you didn't get any errors while adding it and had to try many times >.<).
I would contact support and ask them directly.
For these things, customer support etc, trumps forums.