What Causes Brownish Watery Vaginal Discharge?

I have been having a heavy brownish watery discharge that smells very badly for four days. I first thought that it was some kind of UTI because the first night it was painful to urinate, however that went away. I've also been feeling very lightheaded and weak at times. I'm a virgin so I know it's not an STD or anything like that. Should I be worried?