Nausea is a very common side effect for Plan B.
You need to be careful when worrying about "pregnancy symptoms" after you take Plan B. The massive amounts of hormones can have some pretty negative side effects and they can be similar to what most women consider classic pregnancy symptoms.
Wait another week then test again. Your periods will be a little irregular for awhile and you may experience some spotting between periods, so its hard to say exactly when your next period will come-- everyone is different.