What Causes A Yellow Penis Discharge?

i had a yellow discharge from my penis.....i went to the clinic n got a std screening...they swab me....gave me a shot and made me take 4pills....the doc said i should be ok but i have to wait 2weeks 2get the results from the test.....but i still have a discharge every once n a while.....no buring when i urinate though....whats wrong???