What Causes Delay In Menstruation Other Than Pregnancy?

I was just wondering I am about 10 or more days late on my cycle, I have taken two pregnancy tests and they both came up negative. But i feel bloated ( or that i ate to much) constantly the past week, and in the afternoon i get nausea. As well i have been very tired and can sleep for 10 hrs or more. Im wondering if the home pregnancy test could be wrong and if i should be going to see a doctor asap.