What Causes Delayed Periods, Cramping, Dark Color Urine With Negative Pregnancy Test?

Hi I am 25 years old and always had regular periods. I am eight days late, my last period was 27th of April. I'm having some early signs of pregnancy, cramping, tiredness. Feeling sick, my urine is really dark in colour I have done 3 test which where negative. Could I be pregnant or is there any other explanation