10 days late period with pains in uterus.

My period was meant to start 10 days ago, but hasn't yet. I also get pains in my uterus as though I am on my period and also some sharper pains along with them. I have taken two pregnancy tests which were both negative. Can anyone help me understand what's going on? Another thing I should add is that I have had quite alot of stress over the past week. Maybe that could be what's causing my problem?

Thanks.