I have a pimple like lump inside my belly button. it stings when i touch it and it might be infected. what should i do?

Depends.: It could be a small pimple or it could be a small hernia, so an exam would be best. If you have fever, nausea, a general feeling of being ill, it is urgent to see a doctor. A small pimple or reddened area of 1-2 cm could be treated with warm compresses to bring it to a head, if larger, it should be examined. If just skin irritation, then topical antibiotic cream is ok.

See a doctor.: It sounds like it could be an infection and may need treatment. Only an examination by your doctor will give you the correct diagnosis and then the correct treatment.