Chlamydia produces a white discharge (gonorrhea a yellowish discharge), but you've tested negative for both of them. You should get tested for NGU and a urinary tract infection when you get the chance, but this is likely nothing to worry about. At your age, prostate problems are not likely (especially if you don't have any other symptoms). If it happens only in the morning, it might just be precum from nocturnal erections or a mini wet dream.