Relax. It's actually been there all the time, but you never noticed it until you hit puberty. It's called the "Raphe line," and all men have one. Some are more obvious than others. This is one of those little-known but miraculous sex factoids.
It is the result of a fetal developmental phenomenon whereby the scrotum (the developmental equivalent of the labia in females) and penis close toward the midline and fuse. All male mammal embryos initially start off as female. You might say “female” is the default setting. It takes something extra (androgens) for males to differentiate as male around the sixth week of embryonic development. Once androgens (most notably, testosterone) kick in, the vagina seals up and your penis and testicles form. That line is the scar left behind when everything seals up. BTW: all birds and reptiles are just the opposite: male is the default setting.
So you see, whether biologically female or male, we're all the same under the skin. Dr. J
get it checked by a doctor