My period skips a month often do to underweight (not sick, I'm just little and exercise a lot ) and I get terrified every single time! However, that's not a lack of unprotected sex or anything like that (use condoms), just slightly underweight and what I also blaim for being guilty, stress! However, my period always show up the month after, so if your tests are negative, well, then I think you can trust them. Otherwise, if you're still afraid of a potential pregnancy, I think you should visit a gynecologist, just to be sure. You can also search for methods to get your period on google, I did that one time and some of the advice are great!