What Causes Delay In Periods?

i am a 28 year old female .. i am having high sings of being preggo i am 11 days late i an sick and i am sleeping more .. i have taking so many hpt and they all say no... about 4 years ago i was hit bye a car and had alot of surury did to my stomach i had a part of my coailing removed and ect.. ...note that i am 11 days late and my period have been coming right on tie