metalhead may be correct. stress is a common reason for our cycle to get thrown off. changes in diet and exercise are another. or if you've changed birth control recently. going from one pill to another can throw your cycle off. or if you've taken planned b.
or maybe you are pregnant and it's still just too soon to tell. 7 negative tests when you are only 9 days late doesn't really mean much. it's usually recommended that you wait at least a few days, even up to a week or so before re-testing. and also be sure to use your first morning urine when you re-test again, if your period still hasn't come in a week. if your period hasn't come within another week, and the test are still negative and you're still concerned, it might be a good idea for you to call your just to make sure everything is fine.
btw, breast tenderness can also indicate pms before your period.