My first time taking Plan B?

From the dates you've given, you were likely fertile, & possibly actually ovulating, when you had sex, & if you were within 24 hours of ovulation, Plan B may not have been the best choice of emergency contraception.

The only emergency contraceptive pill proven can prevent pregnancy during the 24 hours around ovulation, is Ella (Ulipristal Acetate), & even that's not guaranteed to prevent it. Plan B might work, but it has not been proven.
You can see that on the Ella website (that it's the only one proven to work during the 24 hours around ovulation).

Plan B is not as effective as regular hormonal birth control.

That doesn't mean you're definitely pregnant. Not everyone becomes pregnant just because they had sexual intercourse when they were ovulating, & you may not have been ovulating. It's just that from the dates you've given, I'd say there's definitely a chance.

Some pregnancy tests will give you an accurate answer from the day of your missed period, if you want to try one of those. Late periods are common after Plan B, though. Sometimes they come on time or just a week or two late, & other times women can have irregular periods for months afterwards.

The symptoms you're experiencing are also common after Plan B.