Okay, so.. this particular thing has only happened to me about 2 or 3 times, once today. But here goes.
On these few occasions, I would go to urinate, and everything would go fine until near the end, where the stream starts slowing down. I'd feel this weird (what I can only describe as-) spasming feeling behind my penis where the sphincter is, and then the urination stops. But then there's this burning sensation in my urethra, sometimes in one place. I feel like I still have to pee, but nothing else comes out. This feeling persists from a few minutes to an hour.
Does anyone have any idea as to what causes this? For reference I am 18, I don't drink or smoke, I have a healthy sex life (with one person, and no STDs), dont masturbate much anymore and havent lately.