I Suddenly Received A Moderately Painful Lump On The Bottom

I suddenly received a moderately painful lump on the bottom of my foot, directly behind my pinky toe (not beside). It is under the skin but above the muscle. It's always slightly painful but if touched or disturbed in certain ways it sends a moderately sharp stabbing pain. It is not a muscle because it will move freely with the skin. It devolved suddenly with a sharp pain. Felt close to stepping on a tack. I've had the the thing for about a month now and the pain is progressively getting worse. I'm not an athlete and I've wore the exact same walking shoe brand and model for over 5 years. I never run and I'm not obese. What could cause this sudden pain?