What Causes Clear Discharge From Penis In Early Morning?

Hi I masturbate very frequently say at least one to two times a day and I do not get my penis out of my pants but rub it with my hand from top of the pants. I am getting a clear discharge in the early mornings from past 3 weeks. I had sex before 6 times with escorts (all of them from legal establishments the last being more than a year ago, but I used condom every time and only once I licked a vagina of one of them, I got tested twice for STI and the results were negative. Do you think I might have STI? Please help