Hi!
Plan B works by delaying ovulation, if you take it before you ovulate. It sounds to me as though you had already ovulated when you took it, and that is why your period came right on schedule. It is possible that Plan B is going to cause another period--because as Wonder4 notes above, it contains progestins. And the symptoms you are experiencing could be PMS--again. We often use progestins to induce a period when a woman has missed hers. It is very common for Plan B to cause irregular bleeding, even into the next cycle. If you have any doubts or worries, take a pregnancy test, but it sounds like you should be OK.
Good luck!
Dr B
Not a Doctor or an expert, but....
After researching and researching Plan B I noticed that it contains "progestin." Some of the side effects listed for progestin include some of the symtoms you are decribing. Not all, but some.
However, as someone who has worked in the mental health field, I can tell you that when you start worrying over something such as pregnancy, the anxiety drives you to start looking for preganncy symptoms. Sore arms, legs, muscle spasms, cramps, hot flashes all over your body...these are anxiety symtoms as well.
When you start looking on the internet for medical diagnosis, sometimes you only look for symptoms that support what you are worried about.
Point is, your worrying might just be what is causing you to have these symptoms/feelings. If you took Plan B as you described above, and your period started when it was scheduled, you are most likely fine. Relax. :-)