Thanks for the query.
From the history you have furnished and on viewing the image attached, it looks like an acne that is secondarily infected. It can also be a heat boil or hair follicle infection.
Kindly do not fiddle with the pimple or try to break it.
You can take a pain killer like NSAID or paracetamol. You can apply a local antiseptic ointment like neospirin over the area.
Consider consulting a dermatologist for an examination and specific medications.
Hope this was helpful.
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