could it be the muscle that controls your penis? If it's not any of what I asked then you might be safe, but if i were you I would arrange a time to go see a general doctor and get checked out specially if it gets bigger over the week.
This happened to me when I was 14. In my case when I went to see the doctor he said I had a slight hernia from picking up to heavy weights. He told me to stop with the weigths and see him in a month. That was over 40 years ago and never went back but never did weights again. The only way to know for sure is see a doctor.
