What Causes A Thick White Vaginal Discharge?

hi dr, im 29 years old and mother of a 4 years old. my problem is that i have a thick white viginal discharge that smells a lot. it doesnt itch. i have been to the dr a few times and they did the test but everything came out negative. what can be the reason for it. really stressed because of this. have to change my panties several times a day. many thanks in advance for your reply