Guys, I'm getting scared.

So I've never been late before, period wise. My period is now 8(almost 9) days late, and theres no signs of it coming. I'm getting worried because I've taken numerous hpt's, got one done at the doctors office, got a blood test done. Everything came back negative. Im not pregnant(did this on day 5 of late period). I had really bad stomach and back cramping today for about 30 minutes/1hr, I have no idea why. I didn't need to poop, I tried, not constipated or anything. But if I'm not pregnant then where the heck is my period?