I am not much help with the human aspect of this but I actually just had this problem with my dog. There was no bone holding one of her teeth in. They called it a bone abcsess. Talking to the vet they said it can be caused by chewing on things that are too hard, and it messes up the jaw bone and it can cause it to rot away. It can also be caused by a rotten tooth, if the decay spreads to the bone, it can cause this issue. Since the root canal wasn't done properly I would assume that either this caused it, or the damage was done before the root canal.
My dog had the same issue, the pus and infection underneath the tooth. I would talk to your new dentist about it because I would say there is a good chance the old one could be at fault.
Seeing an endodontist is advised.
I took my daughter to the Endodontist yesterday. He started the root canal on both teeth. He said the original root canal wasn't done properly. This is why the infections kept coming back and now destroyed her other tooth. They are watching another tooth because that one may go too. I was told that using a bleaching agent on a poorly done root canal can cause severe issues over time so my daughter may lose her teeth from it. She will need to keep her braces on since there is no bone holding her 2 teeth in place. She was due to get them off this month but now that is on hold. To a 13 year old, that is huge but she also understands that if she had them off, two teeth would be missing.
Thank you for everyone who commented.
General Suggestions
Gravity boots or yoga are better than a chiropracter for putting your spine back inline if thats the problem. An aspirin sized amount of calcium powder rubbed on the gums will provide very rapid pain control. Too much at one time will give you headache. Rinse mouth with warm water with teaspoon salt in it or clean seawater if you can get it. Oregano tea either cold or hot can take of infection. Oregano oil is also available. Chose this before taking antibiotics. Place a teaspoon of virgin coconut oil in mouth and swish it around as long as possible also good for dental problems, called oil pulling
Don't pull your teeth out or you'll be more sorry. Abcesses can be relieved with determination. Stop eating and drink fresh green vegetable juices. Eat fresh green cabbage. Blend half a small onion with one stick of celery in a cup of water. Drink slowly, swish around in mouth.