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.
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.