Late but no positive test

Ok ladies, Im not sure what's going on to be honest. I have a period every other month normally every once in awhile I will have 3 periods and then skip a month and go back to every other month. My problem is that Im over 2 months late and I have taken multiple tests and even gone to the women's clinic near me for a test. So since Im not pregnant does anyone have an idea on what might be causing this?