Hi and welcome to HCM,
Infected mandibular teeth often pass the infection to spaces of neck and the lymph nodes . Enlargement or growth of the lymph nodes is lymphadenopathy
which can be caused due to bacterial or viral infection and it is seen with fever
You did not mention the tooth where RCT was done , if it is a posterior tooth of lower jaw that we can suspect the formation of lump in relation to tooth.
If the swelling is related to tooth ,use of antibiotics will not show expected results as the tooth needs to be re opened and explored.
exact diagnosis can be made only after clinical examination and radiographs so I suggest you to visit a dentist
and get a x ray
done for the area , which will help in showing the extent of the lump and its relation to the tooth accordingly further treatment can be planned.
i hope this helps you , get well soon.