There are some types of "normal" discharge that may excrete from the penis. Semen, urine and pre ejaculatory fluid are all normal causes and may be the result(s) of this. The more specific type "abnormal" conditions (STDs) which may cause discharge from the penis are chlamydia, gonorrhea, and non-gonococcal urethritis and others. Because some factors which may cause this may indicate an STD and is not considered normal, I would suggest that you get re-tested for your health and safety concerns. This also may be found to be a normal condition for your body and not STD related. Many STD's are treatable and curable with the right treatment, and sometimes may require more than one run of antibiotics, or multiple doses to treat properly. For your safety, I would suggest that you re-visit your physician and verbalize your concerns again. It may also help if you are having the symptoms (discharge) to show him. he/she may also test the fluids for more accuracy and diagnosis. It is better to be overly safe and get tested for your health and others.
I will also include a website below for further reading if needed:
Penis Discharge - Queensland Health
I wish you the Very best!
If you have any further questions or concerns, please do not hesitate to ask.