What Causes Yellowish/white Vaginal Discharge?

I'm 21 years old, but for the past couple of years I've started noticing an excess of yellowish/white discharge on a daily basis, enough to be annoying. Also, sometimes before but mostly during bowel movements, I noticed a goo-y flow of yellowish/white vaginal discharge. I use the lowest level of birth control so I see my gynaecolologist yearly; nothing has ever come up from pap smears. I also have had a boyfriend for 3 years so the risk of an STD is basically impossible.