What Causes Missed Periods With Negative Pregnancy Tests?

I have missed my period since sometime in late april or may. Have taken pregancy test many times over the months and all have negitive, i also check myself for yeast infections and that too was negitive. I am getting concerned about it because some of the women in my family have had health issues and scares down there. do you think you may be able to tell me what may be going on with me why I am missing my period? any idea or any infomation you can provide would be helpful.