Hello
Thanks for posting your query at DoctorSpring.com
I do not think you are pregnant.
This is because -
1. You had your period on March 20, and the sex was on April 12, 23 days after your period, when you are least likely to be ovulating. This is assuming you have regular 28 - 30 day cycles.
2. You used a condom, even if it slipped off, it still protects you to an extent.
3. Plan B is more than 95 % effective in preventing a pregnancy when consumed within 24 hours of unprotected sex
4. Plan B itself can cause a delay in your cycle , causing it to be late by a week or so.
So I think the delay is due to the Plan B.
Please take a pregnancy test if you do not have your period within a week more.
All the best
Please feel free to discuss further.