Sharp knife stabbing like pain

Every month it seems I get this strong, knife stabbing pain in my right side of my ovary. It comes and goes. Sometimes last a day.. Well I'm going on 3 days. It hurts to put any pressure towards that area (like leaning forward and trying to touch my toes, or sitting up and propping my feet up). My glow account says I'm supposed to start my period yesterday, so far nothing. I'm debating on wether or not I should go to the ER but this shit is excruciating!