The Plan B pill is only about 75-80% effective. It's a massive dose of progestin hormone, I think...and it can throw off your cycle and make it unpredictable for up to a month, as well as give you other symptoms that are similar to menstruation cramps and/or pregnancy symptoms. You can get these symptoms/side effects whether or not the pill works and you do or don't get pregnant.
The pill is most effective if taken correctly in less than 24 hours after unprotected sex, which you did.
However, it's not a guarantee you won't get pregnant. I used the Plan B pill after only 10 hours of unprotected sex and still got pregnant. I have a happy, healthy 10½ month old baby boy today to show for a $50 dose of Plan B, lol.
The only way to find out is to take a pregnancy test when your period is due.
Sometimes it doesn't work. My friend use it and she ended up PG. BUT she did end up miscarrying
Probably did but I would take a home pregnancy test to be sure. It sounds like to me your body is trying to regulate itself from the plan b. Good luck