Help with Macy's/UCB Credit Issue

So i had a Macy's card, which i used for a very small purchase (under $10). When i went to check my balance online (at Macys.com) my account indicated a balance of $0. I have several credit cards and am always very timely with my payments, so i figured i had paid this small balance. Even to this day, the online account is still showing as $0, so i'm pretty sure there is something wrong with the functionality of the online account. The account was with Citicards, and is now being handled by UCB. The balance is now up to about $138, based on a series of late charges. I have sent Citi a letter detailing my issue and they haven't seemed to care. UCB is calling me non-stop and has been almost threatening when i've spoken to them. I feel like this is an outrage, and i am hoping someone can give me some advice as to how to solve this problem.

Thanks!