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If you have a lump which is painful on you inside of heel you need to look for:-
-if there is redness
;swelling which is soft it could be a abscess
-if the lump is hard it could be a ganglion
or a overgrowth of a bone.
In either case it is better to get investigated so get an X-ray it will show the difference if it is a bony growth or a abscess.Once the diagnosis is made then you can consult a doctor who will prescribe the necessary medicines;till then to relieve the pain you can take a painkiller if you are not allergic to it.