This is not a chargeback! Fortune Affiliates have negative carryover! I saw this thing a couple of months ago and I'm pretty sure that this wasn't in their terms and conditions back in 2009 when I have registered with them.
Look back in your stats, a chargeback was likely added to the month before. The money can sit there for quite a while before a chargeback comes through, and they take it from the month that the deposit was made.
My January stats show $0 in Commissionable Revenue when it was actually about $4k in the hole (it never shows a negative in that column), plus a chargeback that showed up a couple weeks into February. The only thing that carried over was the chargeback.
The number they base your commission on (Commissionable Revenue) is never negative, so there is no negative to carry over, unless there is a chargeback.