Can I still be charged interest while I am being garnished?

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I live in the state of Pennsylvania. I have a credit card that went through collections. The credit card balance was around $6,000 whenever it went into collections. I received a call from the company saying that i was to pay the full amount, i told them i didn't have that kinda money and i could do monthly payment. They told me I would have to pay half the balance to be able to do monthly payment. Then on my paycheck it showed that i had a total of around $9,000 2 years later? So I've been garnished for 9 months and when i went to get my taxes back and saw that i was not getting anything back saying that the money went to the garnishment, I called again asking about the tax money taken and the balance was not reduced to the correct balance, i was told that i was getting charged for interest WTF? I never knew you could get charged interest when i never been to court for the garnishment. I was wondering if they can really charge me interest while being garnished ? I called them and asked them for a receipt so i can see how much i have paid so far and how much interest they're charging me but they never did and they said they would... Is this legal at all? I know i made a mistake for not paying my card on time. I was young and stupid and i forgot about it but that doesn't mean they have to rip me off like that.

Can I still be charged interest while I am being garnished?