What Causes Delayed Periods And Negative Pregnancy Test?

I am 40 yrs old my son is 12yrs and daughter 6yrs, I had regular period Dec 16 2010, Jan 17 2011 and Feb 16 2011, I miss my period since Mar till today April 7. since my last period till end of March we had trying to have bady we had unprotected sex and my husband release his sperm inside my vagine everyday. On 28th March 2011 I had blood test & ultrasound scan shown not pregnant, and on 5th April 2011 I had pregnancy test strip shown I am not pregnant. May I know why I had no period and all test shown I'm not pregnant?