Give it another few days, until about 10 days late. Then retest - a urine test.
More than likely you are experiencing NORMAL side effects of Plan B. Your next FEW periods can be early or late and heavier or lighter than normal.
It is not early to test. You can test as soon as two, preferably three, weeks after having sex. Use ONLY your very first morning urine, wake up pee, when you test. It DOES make a difference in the accuracy. Your morning urine is more concentrated. Also, use a name brand test, not a dollar store version.
If still nothing, it's time to see your doctor. Again, this is common after emergency contraceptives and it is unlikely you are pregnant.