What Causes A Pea Sized Ball Under The Skin On The Penis?

Hi, I am 18 years old and I am starting to worry about my penis area. I have not seen a doctor about this so you are the first to hear about it. I have a pea sized ball on the left side of my penis. The ball is underneath the skin just above where the penis starts to go into the internal parts (the opposite side of the head). I was tested for STDs and AIDs last year and all the test came up negative.



Please I could really use your help so I can figure out what I need to do.