I skin was almost completely clear for the first few months of 2014.

Then I took Plan B at the end of April, and my acne has been back ever since.

I'm assuming it's related, so I'm wondering if anyone else has had the same experience, and if so, how long it took for things to go back to normal.

Yes, quite possible. There is a thread about Plan B on this forum, search for it to read all the posts.