It depends upon whether there was fraud in the inducement to get you to enter into the agreement. If so, then you must do something about it within a limited period of time, called the statute of limitations.
However, often times the amount is greater than the original amount due to interest or other fees being tacked onto the original sum.
You should check to see which it was and try to contact the service again and try to negoiate with them again.
In the meanwhile you should defend the action against you by hiring an attorney or doing it yourself.
Do you have a copy of the stipulation of settlement? If so, review it and see that it comports with the payments you are making. If you do not have a copy, go to the clerk's office of the court where the case was heard and order a copy.
If the settlement allows for interest and fees (and those words will be in the settlement terms), it is possible that this is why you are paying more than the stated numeric amount in the settlement. If the words "interest" or "fees" are absent from the stipulation, it may be worth having an attorney or the judge who approved the settlement take a second look.
Although collections lawsuits tend to be for small amounts of money, it is always a good idea to hire an attorney to represent you and assist you in either defending or settling these actions. My firm helps with the defense of credit collections lawsuits.
Best of luck.