How long does plan b affect your cycle?

I took plan b for the first time back in May. I had no symptoms & my cycle wasn't affected at all, I got my periods on time. I had to take it again almost 3 months ago & my initial period after the plan b came right on time. My next period after that came about 2 days late.
Now I am on my third period since taking the pill & my period is late yet again. I have had sex in the past two weeks. However, the first time I used a condom & the second time I didn't because I thought I was in the safe zone.
 Anyway I can't imagine plan b affecting my cycle this long. So, my question is has this ever happens to anyone? Can plan b still be throwing my cycle off even though I'm on my third period since taking the pill? & when should I take a pregnancy test? I'm freaking out & any help would be great!