What Causes Delay In Periods And Abdominal Pain?

Hi, I'm on time every period but this month I wasn't. I was due and had no symptoms which is very strange, I had slight red discharge but a pad on then nothing. I have done two pregnancy tests and both negative. I was due on 26/5/11 and yesterday and today I have had pains on my left side which feel like ovulation pains but I thought that it's too early for it to be that. Please advise as I'm confused and have never missed a period thanks