Late Period but Negative Test

I am now 13 days late but took a test after being 7 days late and it was negative. I'm a collegiate runner, underweight, and have only 9.2% body fat due to a health condition but my period has never been this late before and I'm already paranoid about pregnancy. Do I take another test or wait it out because it could just be that I'm so thin? 

Vote below to see results!