What Causes Delay In Periods And Intermittent Cramping?

I have been trying to concieve for the past 6 months with no luck. My period is usually every 40-43 days long. My last period was December 22, 2009. I should have gotten it in the first week of February. I am now over two weeks late and pregnancy tests have come up negative. However I've been having some cramping, off and on. What could this mean?