Debtisgood has a valid point there...you may have lost some history when the CO fell off. It may seem counterintuitive, your score DROPPING when something BAD falls off. But if that was your oldest account, losing it will cost you some history.
You may also have been rebucketed, as FICO scoring grades on a curve and you may have moved from a bucket of people with a negative entry to a bucket of consumers without.
In any event, your score should increase from here, over time.