What Causes Irregular Periods After The Treatment For Vaginitis?

I Have A Very Important Question...I Usualy Have Normal Periods That Come Every Twentyeight Days, Over The Past Few Months They Started Comming Sooner And Lighter Then Late And Light And So Forth. They Recently Started Being Normal Again On The Thirteenth Of Every Month. Now This Month It Came On The Nineth And Was Lightish And Such, I'm Worried Because My Doctor Cannot Tell Me What Wrong With Me. My Doctor Has Done Tests Upon Tests And Has Found Nothing. I Was Recently Treated For A Vagnosis And It Helped The Pain A Little But She Told Me I Could Be Sterile From It. I'm Worried About That. My Fiance And I Have Been Having Unprotected Intercorse For The Past Nine Months And We Have Not Been Trying To Get Pregnant But We Have Done Nothing To Avoid It Either. Can You Help Me Out?