Do you walk alot, if so, this could be an ingrown hair more than likely, especially due to friction form sweating and walking a lot.
It will probably go away on its own but it will take time. Put warm compresses on the area, it should bring the puss to the surface and you may be able to squeeze it then and find a hair.
You may get an antibiotic ointment and apply it for a few days or a week. If it’s not ana ingrown hair, it may just go down after antibiotic ointment.
Try different stuff.