10 Days late UPDATE. HELP!

Hey ladies, so I am about 10 days late on my period and I'm not sure why. I took a pregnancy test and it was negative, but does anyone know what could be causing this? Any advice?
So I still haven't started. I don't have any symptoms of pregnancy, but my stomach has been hurting literally every day for a couple weeks now. Could that be related? Should I see a doctor? I'm starting to get really worried. I am now going on 13 days late.