Vagina itching

2 weeks ago I started to get an itch it's the outside of the vagina entrance and a little bit in the entrance....I'm not sure what it is, I have been to the clinic on the 4th day when it happened and I have been test for sexual transmitted disease but my result are 100% negative....I'm not sure what might be causing it, it might be my underwear I have changed which helped for a couple of hour and goes back to normal I wear cotton girls short underwear I'm not sure if it the material or something but I have been wearing the same style for 2 years....if anyone has any tips and tricks please help