What could this weird pimple thing have been?

Im a 19 year old guy, and about 2 weeks ago i noticed this purple-ish pimple under my left nipple. It was a little bit bigger than a normal whitehead though. I waited to see if it would go away on it's own and it didnt after 2 weeks so i tried to pop it. It did, and there was some blood and puss and stuff but there was also this small hard black pebble thing in it. Any idea what it could have been?