How common is vagina irritation an burning from sex and underwear been this way for few months sex minum because of this any ideas?

Vaginitis...: It sounds like you might be having some type of irritation which could be caused by a yeast or a bacterial infection. In this case, since I have no other details, I would say that if you are also having itchiness as well you can try Monistat or Gynelotrimin (clotrimazole) over the counter. However, if severe or no response see a gynecologist for exam and treatment.