Since it has been more than 2 weeks since the act of unprotected sex then a pregnancy test would be accurate. If they are saying negative then you are not pregnant.

Plan-b can mess up your cycle along with stress.

If you are getting negative pregnancy tests, than I'd be pretty positive that you are not pregnant. Eiri is right... plan b definitely can delay (and in some cases, bring on) your period. Stress is always a likely contributer as well. Just try to relax with the fact that you know you are not pregnant, and your lovely period will return to you. Make sure you use condoms and/or hormonal birth control (pills, depo shot... whatever) from now on, as plan b shouldn't be used regularly unless necessary. Especially with the side effects (nausea, messing up your cycle, etc), I'd try to play it safe from now on with birth control usage from the beginning.