Root canal on lower jaw 1st molar.now hard, painful lump inside my cheek, next to tooth. what is it?

Needs examination: Difficult to say - may be an ongoing infection or simply inflammatory tissue or some other issue such as salivary gland infection. Needs fairly prompt dental or medical examination (GP or urgent care or urgent dentist) and right away if breathing or swallowing difficulties. All the best.

Dental abscess: It is likely that you may have developed a tooth abscess after the procedure. I would recommend a visit to the Dentist once again to see if the abscess needs drainage. If you are unable to visit a dentist, please visit your GP or Urgent Care clinic to see if you need to be started on oral antibiotics.

See below: See the dentist that performed the root canal. Variables include, need for antibiotic therapy, incision and drainage,, incomplete fill, separated instrument, additional canals. See the dentist and start with the simplest first.

Hard painful lump: Most likely an abscess. You need to be seen asap for definitive diagnosis and appropriate treatment. Dental infections can spread quickly and cause serious complications. Please don't wait.