Periods

I am 20 years old. Sexually active, never use a confom. I was diagnosed with PCOS about a year ago. I take birth control pills every day and have been on them for years. Recently I have been having 2-3 week periods and even visited the ER recently to see what was going on. My gyno says I'm fine and the ER doctor said my levels were normal... What could be going on with me..? does anyone know?