Could be a lot of things. What have you done with your hands over the past few days? Could it be a small thorn or splinter that got a bit infected. It could be a spider bite. It could be a cyst or even a pimple or an in-grown hair. If it bothers you and you can’t figure it out, go to a doctor. If you just want to get rid of it, try applying hot compresses and use tribiotic ointment.