Yea, dude, you're going through puberty. We all go through this.
Do you already discharge semen when you masturbate or no? Sometimes this happens to your body during puberty when it's changing and getting ready to be able to discharge semen when you orgasm. If you already have shot semen out before, it probably is pre-***, a substance that can come out of your penis just from being excited (and it actually has a tiny bit of sperm in it and can get a girl pregnant). There's a 90% chance that this is just normal.
But you also can discharge out of your penis if you have a disease. Have you had unprotected sex with anybody? If you haven't had unprotected sex, I would not worry about the disease at all. It's most likely just puberty and the normal changes all males go through.