What Causes Delayed Periods And Negative Pregnancy Test?

I am 13 days late and since the age of 10 (I am 22 now I am about 5'5" and 165 lbs), when I started mentrating, I have had a very regular 28-30 day cycle. I keep track. Me and my boyfriend did have unprotected sex and i am not on any birth control. I also have been urinating frequently, I have had cramping on and off for the last five days and about a week and a half ago I felt extreme cramping that lasted only a few minutes. Plus I am very tired even though I get plenty of sleep. I have take two home pregnancy tests and they were both negative. Is it possible that I am pregnant?