Hello and thankyou for approaching DoctorSpring with your query.
I have gone through your case and understand your concern.
The bleed would not be due to prostate cancer as prostate cancer develops in older men after the age of 40.
If there was blood in your semen it could be due to minor trauma or due to infection or stones in your genitourinary tract and it should subside by itself.

Please can you answer a few questions so that I have a better understanding of your case.
1) Did you have severe excruciating pain and immediate loss of erection along with the bleeding?
2) Is your penis discoloured?
3) Have you had sexual encounters before?
4) Do you have any burning while passing urine, frequent or painful urination?
5) Do you have any urethral discharge?

If the answers to the above questions are no, you need not worry. It would be a benign condition and will subside on its own. However if it repeats, you will require to get the following tests.
1) Urinanalysis
2) Semen analysis
Depending on the tests results further testing may be required.

I hope this has helped. You can reply as a follow up.
Thankyou