Small hard swelling and stuck mouth after 3rd molar extraction

I had my lower left third molar (my last wisdom tooth) extracted 15 days ago. The extraction took 45 minutes, there was a lot of poking and extracting pieces of the tooth and roots. The tooth was fully erupted and developed, but had bulbous roots and just didn't want to come out in peace. There was no infection and pain, but there was some decay in the crown. A tiny tip of one of the roots fell somewhere inside during the extraction and the dentist couldn't find it and left it there.

First three days were ok, I managed the pain well with Tylenol and cold compresses, ate soft food etc. The site healed very well and is now basically closed, pink, no signs of infection, but I had swelling along the mandible that has now contracted into a hard ball under the mandible, approximately at the level of the sixth tooth. It's not painful. The problem is I cannot open my mouth all the way. I can stick my index finger in between the upper and lower incisors and that's about it. Trying to open my mouth more is extremely painful, the pain is located mostly in the inner angle of the mandible. I can move my mandible from side to side, but not up and down. Warm compresses and rinsing my mouth with saline doesn't help.

Never had any trouble with the previous three wisdom teeth, including the lower right one. I'm 45 yo, female. What is this and what do I do? The swelling's been the same for the last three days and my mouth is not getting any better...