Currently available tests for sexually transmitted infections are among us the most sensitive test in all of medicine and with two negative test you can be absolutely, completely confident that you do not have any of the STI’s tested for. Further, the nature of symptomatic STI‘s is it symptoms do not come and go as is your experience, but tend to be present continuously or even progress over time.  Normal urethral discharge varies substantially in quantity and nature, affected by variables such as duration since last urination.  In addition, straining at the time of bowel movements tends to express prostatic secretions normally.  Your hyper vigilance may be contributing to your observed increase in urethral secretions.