You wrote, "I paid a co-pay out of our joint account. My husband and I recently separated and got back together but we kept our bank accounts separate."
A joint account is not keeping your accounts "separate."
I suspect you have three accounts at the same bank and that the bank has mishandled your accounts.
In most states, as long as you are married, you are probably responsible for your husband's medical debts. You may want to check the court records to see how this happened, but my guess is that a lawsuit was served on your husband naming you and that your husband failed to tell you about the suit. That would make the court judgment perfectly legal in most instances. Hope this perspective helps!