It’s fairly common…
Collection agencies do this all the time. What happens is that the medical provider sold the debt to the collection agency, which reported it automatically. It’s normal for collection agencies to report debts even without calling debtors. Them reporting has nothing to do with your request for documentation.
Are you sure that the debt isn’t yours? It’s very likely that it is a medical bill your insurance should have paid or you thought they did. Pretty common for unpaid medical bills to get sent to collections.
If it turns out that it is your debt, try negotiating a pay for delete agreement, where you pay and they remove the item from your credit report.
If it’s not yours, dispute the item. You may need to repeat the dispute for all three credit bureaus, but first check all your reports.
In any case, since only a month has passed I’d give them another two weeks to send you the documentation. If they fail to provide proof – proceed with the dispute.
Good luck