It looks like you are having periapical abscess
. this happens when the decay has become deep and reached the pulp and progressed to tip of the root. the pus is now probably draining out through the sinus which you have described as lump. since the X-ray has been taken, it would clearly show how deep has the decay reached and whether that is the cause for for your lump. in your case most likely root canal treatment
the second possibility is that the tooth is decayed beneath the filling and the tooth has chipped off leading to food and plaque
accumulation, which might have caused gingival abscess
(abscess of gums ) . in which case re doing of filling is sufficient provided the decay has not reached the pulp (X-ray will confirm this).
in either case you need not have to be scared, dentist
will anesthetise the tooth during the procedure (if he doesn't then insist on it).
visit dentist regularly and get yourself examined to prevent future problems.