What Causes A Delayed Period With Negative HPT?

i am 2 wks late and 3 negative hpt,i feel a "ball"where my uterus is located...what's going on...could it be a false negative, could it be a tumor where i thought i feltg my uterus.i have 2 other children so it is possible my uterus would be "showing" sooner.haven't gone to docs yet