What Causes Severe Vaginal Discharge?

I'm 24 years old. I have always had normal amount of vaginal discharge. But recently it has come to a point where I am using pantyliners. The colour varies, from being whitish to being yellowy. There is not a strong smell but enough for me to notice it. I'm still a virgin and I am slightly underweight. Would you advise me to go to the doctors? I am pretty embarrassed about it all. Would I be able to get advice/treatment over the counter?