Plan B is a substitute of the human estrogen called levenorgesterol and prevent the implantation of the fertilized ovum on the uterus. As you have already taken a urine pregnancy test and you have already had you periods its unlikely that you are pregnant but the chances of ectopic does exist. It may cause irregular bleeding or spotting, heavy or prolonged bleeding, or your menstrual bleeds to stop altogether .This is a part of the contraception and usually becomes better over time.
I would suggest you the following.
1) Buy a Urine pregnancy test kit form the drug store and check if you are pregnant 15 days later. If it is positive, make an appointment for a USG to detect uterine or ectopic pregnancy.
2) Always use a method of contraceptive if you are not looking to get pregnant. You can either use barrier method like Condom or Diaphragm or a hormonal method like Oral contraceptive. Both are equally efficacious.
3) There is a slightly increased chance of thrombosis of the veins hence maintain a good and regular regimen of exercise and eat healthy