First, before even reading everything, if you have to milk your penis to see the discharge, it's not really discharge. Discharge from an STD would be there whether you did anything or not.
Did they test you for NGU or look for white blood cells in your urine? You could have an infection in your urethra from normal mouth bacteria, and that wouldn't show on a gonorrhea or chlamydia test. (Chlamydia isn't spread via oral sex, but since gonorrhea and chlamydia are most often tested for together, I'm not surprised they tested you for it.)
Make sure they did a test to look for white blood cells. If not, go back and have them do it. If they did it, and it was negative, let them know you are still experiencing some symptoms, and see what they say.