Medical Debt collector came back after being paid two years ago claiming to never have been paid - I can’t find proof I paid them.

As the title mentioned I had a medical bill from 09/2016 that was in my name (even though I was a dependent at the time) charge for an ambulance trip in the US (I live in VA but ambulance was in CA). Bill never got paid and went to collections under my name.

Fast forward to 2020/2021 I’m working and I can finally pay some of this medical debt off so I paid ever single one that was on my credit report (some in full some reduced amounts). Back then I had email receipts of me paying them, but low and behold I lost access to that email over a year ago bc yahoo sold to att and Their data went to crap (I tried multiple times but I can’t retrieve the email inbox).

Now in March 2023 a debt collector sent letter just now saying that I never paid the biggest one of them(which if that’s the case why wait 7 fucking years to ask about the debt) I called and said it’s already paid, to which they denied. I ask for a debt validation form and they will send to me this week.

In preparation I’m trying to go through my credit cards and find the amount paid but and turning up unsuccessful.

What should my next course of action be ? Where am I not thinking of this properly?