You are prob correct. Do much masturbation (more so, to vigorous masturbation) can cause irritation to the penis and the urethra. You should stop for a week or 2 to let everything rest and heal!
Could it be a prostate issue? Yeah, it could but the the irritation is more likely. Only way to rule out the prostate is to see a doctor to have a proper feel of it.
Obviously is not an std if there was no sexual contact with anyone other than yourself through masturbation. However, you may have an infection. A clear discharge can be a sign of that. Perhaps your had dirt enter the urethra during masturbation. But regardless, see a doctor for diagnosis. And antibiotics if they agree there is an infection.