Why Do I Have A Smooth Semi-hard Bump Under My Penis?

I have a smooth semi-hard bump under the skin on my penis, on the underside of my shaft under the head. It is not painful or cause me any discomfort and doesn't hurt when I press or squeeze it lightly. I just noticed it and believe it only showed up within a day or night. I am going to have it checked out but worried this could be something serious.