Hey, it's good that you're being observant. I've had this too. If your situation is the same as mine was, then a likely cause is that you are simply tensing up that area (by masturbating) and when that area (the pelvic floor) is too tense, this leakage can happen. If you reduce the masturbation, this should go away. Maybe take some days or a week.
Also, I would stop masturbating (as much as possible at least) and stop looking at porn. And also, to prevent possible injury, be careful to masturbate correctly. I believe the correct way is to move your hand up and down your penis without moving the penis itself. And try not to be over rigorous. Also, do not crush or hit your penis against anything either, especially anything hard or jagged, especially when your penis is erect.
I think Helper015 has some good advice there. I'd like to add that you really SHOULD see a Dr. - don't be embarrassed - #1, if it's a male Dr. he was a teenage kid himself once & knows all about masturbation at that age. #2, he (or she) is after all a DOCTOR & they have probably already heard about almost every kind of sexual problem from young guys, older guys, women, etc., some things you probably never even knew about. Masturbation is so normal & so common they're not gonna think anything bad about it at all, and I can guarantee he/she will not judge you or think you're weird or a perv or whatever! The medical field now regards masturbation as a normal & even a very healthy thing - they'd probably be more concerned about a guy your age who DIDN'T masturbate.
The other reason to see a Dr. is, there are medical conditions that can cause these kinds of things. Too-frequent masturbation can cause a form of prostatitis, the prostate gets swollen & starts to interfere w/ normal urination, so you should not just ignore this. It's possible it will go away on its own (following above advice), but if not, PLEASE see the Dr.! You don't have to tell them in any great detail about your masturbation habits, just tell them what's happening, they may bring up frequency of masturbation or sexual activity, if so, just answer honestly but you won't need to go into great detail, just say how often. Hope this helps...