TMobile refunded my past payment and sent me to collections

It was such a pain to cancel my Tmobile account and settle the remaining balance. I was on the phone with a rep for 2 hours (40 minutes waiting to connect). I paid the remaining balance on the account. She refused to email me a receipt and the store clerk didn't want to do it either (told me to continue talking to the person on the phone) I could not remember the pin that was required.

It has been over a month now. I never received anything from Tmobile. Today I got a letter from Southwest Credit Systems LP saying I owe 99.05 + 50.62 in fees. I checked my credit card again today and saw that the 99.05 "last charge" that I made a month ago was refunded. Why did they refund it and send me to collections???

There is no way customer service is going to resolve this. What is my best option?