What Causes A Sensation Of Discharge During Urination?

hi, i have a problem where i sometimes feel like i have discharge coming out of my penis, but there is often nothing there, maybe a little something once i have passed urine. it comes and goes.i went to the GP she thought was thrush and gave me some tablets in case it was a bacterial infection. and some cream for thrush. that didn't do much. so i had a gonorrhea and chlamydia test, both were negative.this has been going on a couple of months now and am not sure what to do, it is quite an annoying feeling