Hello there ,
Thanks for writing your query,
Lumps seen in the oral cavity can be of various types . It can be due to infective lesions related to tooth like periodontal abscess
or gingival abscess
draining in the oral cavity.
According to your clinical history i suspect your upper tooth which has already got a fillling has got infection that has spread to the palatal area leading to formation of abscess in the form of lump.
exact diagnosis can be made only after clinical examination and evaluation by taking radiographs.
i would suggest you to get and OPG radiograph done which will help in diagnosis and accordingly treatment can be planned .
consult an oral surgeon and get the examination done .
i hope this helps ,