Hello,
As it is growing smaller, it may have been a plugged gland or infected hair follicle (if the lump is near pubic hair)
Keep any eye on this to see if it totally resolves. Do not try to squeeze or "pick" at it as this could cause a secondary infection.
Another consideration is that this could be a STD if you did indeed have unprotected sex.
Should you notice the "bump" does not resolve or your symptoms worsen, include severe swelling, redness, fever or penile discharge, it would be imperative to make an appointment with a urologist for an accurate diagnosis.
Please let me know if you have any further questions or concerns.