From the sounds of it you are dealing with a collections agency and not Petal themselves. Is this correct?
What is the balance on the card? Your limit has nothing to do with your balance. If you are behind depending on what your balance is and late fees, your minimum will increase alot. And not paying it's only going to get worse
You keep stating the same amounts, but please clarify, what is Petal saying the minimum due is now? If it's for the same amount, they may have contracted a collection agency to collect for them, similar to what Amex does. It can be in collections even though you can log in to your account to make payments or speak to customer service. If that's the case, the additional amount is the collection fees from the agency. You can request Petal to pull it back and possibly not have to pay those fees, or they could be non-negotiable now and part of what you owe. The CSR may not have known so you should call and escalate to a manager to find out for sure.