Actually pain after root canal retreatments is fairly common and expected. Even up to fairly severe pain. Just keep taking ibuprofen 600mg every 4-6 hours. Ride it out for the next 3-4 days unless it gets a lot worse. It's a fairly normal part of the process. That said, occasionally (rarely) something is really wrong- so listen to your body. There isn't much that could be wrong at this point other than a worsening infection.
You are correct that the tooth itself can no longer hurt. The tooth itself in fact is not hurting you right now. It is the tissue around the tooth that is bothering you. The tissue around teeth is extremely sensitive and doesn't like to be bothered. Unfortunately, retreating endo does often bother the tissue around that tooth, causing the nerves to get "angry" and send off severe pain signals. Some of this has to do with the material used during root canal retreatments. The tissue may swell up which also makes the nerves pretty unhappy and send of pain signals. And sometimes the bacteria gets pushed into the tissue around the tooth during the retreat, starts reproducing (there might be a lag period here when you have no pain), and bam, a day or two after the root canal treatment is done you have a worse infection than what you had before it. This will clear up in a couple of days, once your body has time to respond and the antibiotics kick in. The body takes a couple days to get it's full blown attack on bacteria going. Just be patient, take vigilantly every 4-6 hours, and hang tight. It'll get better!