Will plan b mess up your menstrual cycle?

Plan B should be taken within 72 hours to be effective, however it sounds as though it did work as you had a normal period.

However, I don't understand the timimg. You ovulate 2 weeks before your period is due (or more accurately, your period arrives a fortnight after ovulation day). You are considered fertile on ovulation day and the preceding 5 days, so that means 14-20 days before your expected period. But your period was after plan b was taken (which was 3 days after sex) "about 2.5 weeks later which was a week earlier then my cycle was due". So you had sex on day 1, took plan b on day 4, your perion arrived 17 days later on day 21 and your period was expected a week later on day 28. So it seems you had sex a week before you were fertile. Maybe exact dates would help.

EC can alter your cycle, and subsequent periods can arrive earlier or later than usual, and sometimes one can be missed. But if you have missed two periods, I would suggest you take a test. Although you have not had unprotected sex since that time, it sounds as though you have had sex, and as no contraceptive is 100%, and sperm can get transferred manually, there is always the possibilty of pregnancy.

I hope this helps as a start