You have normal side effects of Plan B. They usually only last a few days after taking. Give it another week or two and your period should come- if not, take a pregnancy test.
Remember to use a condom or other form of birth control next time- since Plan B is only 80-90% effective and doesn't prevent STDs. Also, start charting your cycle so you know which days you ovulate (release the egg) and those are days you can get pregnant.
In a normal cycle (28 days) a woman will ovulate around day 14- you can detect the time you ovulate b/c your body temp will increase about 1 degree at this time and stay higher until your period comes. The cervix also will change position so sperm can enter easier... but the easier way to know is that you'll have "eggwhite" cervical discharge (kind of sticky, whitish- looks like eggwhites-- it's an important fluid that helps the sperm swim easier). If you notice this, then the egg has been released. So, that means that you can get pregnant and if you've had intercourse w/in 72 hrs of noticing discharge- you can be pregnant and up to 36 hrs afterwords. Sperm live about 72 hrs and the egg is viable for roughly 36 hrs. Going back to that "normal" 28 day cycle- if you ovulate on day 14 then you could actually get pregnant day 11-16.
You need to be safe during sex, but to avoid unwanted pregnancies- use additional protection during the days before and after ovulation.
As far as your symptoms- some of that is from Plan B... the other is probably due to stress.
I teach human A&P