It could be an abscess which requires antibiotics as these types of infections can get out of hand very fast. If it is possible for you to do this you should apply warm compresses to the area, and do sits baths a few times a day. In order to do the sits bath use Epsom salt in warm tub water sit in the warm water for atleast 20 minutes a few times per day. You should see a doctor for an antibiotic. If it gets bigger it may require lancing so try to make sure it doesn't get to that point.
HI,
Painful lumps are more often because of infection. This is more likely to be local infection under the skin and not kidney infection. Hence local hygiene and washes with anti-septic solutions will help. Also anti-inflammatory agents and antibiotics may be necessary. If these measures do not help then a consultation with a Doctor is a must.
Take care and good luck.