I have taken Plan B a few times and it messed up my system for a few days, sometimes a week. It's basically making your body not able to become pregnant - so it really screws up your uterus. Not having a normal period after taking Plan B is pretty normal. The other symptoms though, that could happen too. The first time I took Plan B I was pretty sure it didn't work because I felt like I was going through the symptoms of pregnancy, but after a few days it subsided. If this continues then yes, go see a doctor. But testing for pregnancy can only be done 18 days after ovulation for your body to produce enough hormones to show up on a test.
Although Plan B may have messed up your period making it hard to tell when ovulation actually occurred. BUT - you can never have a false positive pregnancy test, only false negative, so if you are in fact pregnant, it may not be showing up on the test because your body isn't producing enough hormones for it to test positive. Don't go crazy taking tests every day. If you're this worried, go to the doctor or planned parenthood and they will give you a more accurate test.
Good luck!!