Probably there is an infection in the urinary tract (actually, in that case it should be burning even after masturbation. Can you tell me if you are finding yourself in any awkward posture upon waking up after a night fall - A position that is pressing the penis hard? Or the erect penis is kind of "folded" inside the underwear? Get your semen tested for any infection and send me the result through a private consultation. Or pl consult a physician near you.