What Causes Pus Filled Sores On Labia?

I am 34 years old female weigh 180lbs and am having reoccuring pus filled sores on my labia.I itch for 2 to 3 days before the sore appears.My Dr. thought it was staph.Gave me 2 weeks of Bactrim and then it stayed gone for a month. this has been going on for over a year now.He tried to do a culture several times and cannot get a definative answer.Also it was negative for Herpes.After the sore is popped it goes away within a day but always comes back.Out of the last 12 months I have had maybe 1 week a month with no blisters and that is when my period starts that they go away and then return several days after my period is over.Please help me..