Late period, but not pregnant?

[artkween12.livejournal.com]

I posted earlier this week about a late period, possible pregnancy. Thank you to everyone who gave me advice.
I took two home pregnancy tests today (ClearBlue) and both turned up negative.
My period is three weeks late, and I've been feeling sick to my stomach for about a week and a half, along with occasional pain in my abdomen, almost like severe cramps.
My boyfriend says I should go see my doctor, and I plan on doing that, but does anyone have any idea what the problem might be?
I've heard that bad nutrition (like, not eating enough) for extended periods of time could cause your period to stop. It's summer break for me, and this causes my diet to change drastically---fast food, skipping meals, sleeping schedule is out of wack. Could all that have anything to do with it?
I should have gotten my period around July 20th. So it's been about 3 weeks.
Any ideas? I'd like to have some sort of an idea before I go to my doctor.