Hello Since you have STDs tests return negative, but the yeast infection, that leads us to two possible diagnoses which are: 1-the yeast infection can cause symptoms like burning, itching , bad odor in the vaginal area . It can be treated by Antifungal vaginal tablets and creams. A sitz bath can be helpful by filling the bathtub with warm water that goes up to your hips when you are seated, you may then add a mild saline solution or baking soda to the water. 2- An allergic reaction to chemicals, detergent or perfumes: in that case you should know what the possible irritant and avoid it, use a cotton underwears and the using of Hydrocortisone ointment or another anti-itching drugs can help. Hope I answered your question Wish you a speedy recovery Best regards Dr. Noor Farid