I Have What Appears To Be A Very Large Pimple

I have what appears to be a very large pimple 3 inched directly below my belly button. I noticed it last week. I play basketball so I think it may be a pimple, and it's right where the waistband of my shorts are, but it doesn't protrude from the skin, it's "embedded" in a way. Actually, it's quite flat. It hurt a lot when I tried to pop it when I first got it. Now, I can barely touch it due to the pain. The portion of it that touches air hurts to the touch. I really would like to know what this is and how to treat it, because the pain is getting to be a major issue. Thank you in advance!