What Causes Yellow Vaginal Discharge?

Hi I have some vaginal discharge.. it is yellow and doesn t smell to bad, maybe a little different, medium consistency. Not allot and no chunks but definitely a different color than normal. I have had it for about 4 days and it is infrequent. I am 50. Could it be something else besides an STD. I will see my GP soon but it is the long weekend now so I was just looking for an answer.. Thank You.