Account escalated before agreed payment date extremely frustrating and unjust experience

I took out an emergency loan with Wonga, with a clear and agreed repayment structure aligned to my salary date. My first debit was successfully processed on the last day of November, which is my salary date. The next debit was agreed to be on the last day of December, following the same arrangement. Despite this agreement, I began receiving SMS messages on the 15th of December demanding payment. This was confusing and highly frustrating, as I only receive my salary at the end of the month and this was the basis of the repayment arrangement from the start. In an attempt to resolve the issue proactively, I tried contacting Wonga multiple times. Calls go directly to a self-service system that is not helpful for resolving this type of issue. I also sent several emails, only to be told that my email address was incorrect, with no clear or accessible way provided to correct it. Shortly thereafter, I received a call from a debt collector informing me that my account was in arrears. This escalation happened before my agreed debit date and before the arranged payment was due. This is extremely inconvenient, stressful, and unjust. I am actively trying to build and protect my credit record, and this situation has the potential to negatively affect it through no fault of my own. I genuinely do not understand how an account can be placed in arrears or handed over to debt collectors before the agreed debit date. More importantly, where was I expected to get the money to pay on the 15th when my salary is only paid on the last day of the month and that was the agreed arrangement?