This has to be a mistake, have you tried calling them to sort it out? Do you have copies of the payments you made? There's nothing more frustrating than seeing your car being repossessed when you have been keeping up with your payments!
If payments were made directly there should be bank statements to prove the money was paid, otherwise maybe cheque stubs.
In my opinion they are confusing you with someone else, either that or, for whatever reason, your last payment was held up and did not get to them on time. Either way, the polite thing would be to contact you and clarify the situation before sending round the repo guy.
Call the company and state your case but try to stay calm and in control. I know all you want to do is yell but that really isn't the best way to prove your innocence.