This happened to me two years ago. For me I had just stopped ovulating. I let the dr I was seeing shrug me off and treat me like I was being silly for worrying. Af not showing up drove me insane. I felt out of whack and would have bet a fortune at times that I was pregnant. Mum and her sister told me they never showed positive on hpt for all their pregnancies. So as you can imagine I went a little crazy. I needed fertility treatment to help things and I'm so glad that I pushed for more help (even if it was a year after no af). My point, try not to panic just yet but if you go more than three months without af insist on someone investigating things.