This sounds like the forming of a Boil (large pimple). Unlike a pimple the Boil will start deeper and will not form a head until later. Start washing this area many times a day with hot water, acne treatments if you have them. If it continues to grow have someone check it. If it grows to a full Boil you will most likely need a medical office to lance it for you.
A boil is where the skin pore gets closed up and bacteria starts to build underneath. You have to get the pore opened and cleaned.