I paid a Collection debt with settlement. Is my problem gone?

In March/2008 I paid 60% of the huge amount that AT&T was charging by a Collection agency. I did a set up settlement with the agengy. Today 10/09 I saw in my credit report that AT&T put the rest of the amount in my credit report thru different Colletion agency without contact me. I called At&T and they said I own them this money. But when I paid the settlement the Colletion said I was done with this debt and they sent a letter saying that. I'm very upset with AT&T for messing up my credit again. They are asking me to send a bunch of papaer to prove the settlement. I'm affrad this problem never stop. I want to sue AT&T since my bill was $700 and I paid the settlement $2000 (after 2 years negociating with them) of the $3500 that they were charging... What do I do? - Is this your question? Add additional information