It may be a payment dispute, but I haven't bought anything since last month.
Funnily enough, the same happened to me when I tried to logon to my steam at my friends house.
Did you find a workaround? I've tried and tried, I've memorized almost everything about my account by heart, so I know I'm not making any spelling errors.
EDIT: Mom just came in and told me she found transactions from Steam from the month of July. I haven't even touched her credit card.