What Causes A Liquid Discharge From The Penis?

hi i have been geting clear liquid coming out of my penis all day its not alot but is alway there have been to the doctors and thay gave me trimethoprim and did a chec for std but nuthing there ave been on tablets for 6 days na and nuthing its still the same pleas can u help i am 29 and v helthy uther than this it started about a week ago