Sour tasting vaginal discharge?

I noticed that I have sour-tasting vaginal discharge.  I noticed it one day while my boyfriend and I were having sex, I went down on him and I almost gagged because my discharge on him was so sour tasting ... it really took me by surprise. I never had this problem before, and my discharge never tasted sour before except for these last couple of months.  There is no smell to it at all, the discharge is pearly white, not clumpy, and I have no redness/itching nor any other symptoms ... it's just the taste.  I am not taking any medications or birth control, I have been tested for STD's which all came out negative, PAP smear negative as well. Does anyone know why this is happening (seriously, this has taken a large toll on my sexual relationship - I don't enjoy it because I am constantly worried about it and I am getting depressed over this).  Does anyone have any remedies to get rid of the taste?  I have done quite a bit of research and I haven't found much help/explanations.  Please help, I am desperate.  Thanks.