It's
false.
The chances aren't as good but they do exist.
Sperm can be maddenly resilient.
You get pregnant because you're ovulating (see:
http://en.wikipedia.org/wiki/Ovulation#A_closer_look_at_the_process ) and sperm meets egg. However, at least according to Go Ask Alice, sperm can live for up to 5 days (http://www.goaskalice.columbia.edu/0797.html ). I had heard 2 weeks but I am having trouble confirming that.
So, let's say you finish your period on the 1st, this incident occurs on the 9th and the sperm live for 5 days. Well, guess what?! It's possible that you ovulated somewhere in there. So conception is
possible. I'm not saying likely, I am just saying that it is not impossible.
And, ovulation rates vary. It might be 14 days after your period in March, 13 days after in April and 15 days after in May. Add that to the sperm lifetime thing and yes, you can get pregnant 8 days after your period. This is a big part of why the rhythm method does not work well. See:
http://www.kidshealth.org/teen/sexual_health/contraception/contraception_rhythm.html
You can get a home pregnancy test and check. If it's embarrassing to buy one, well, it's going to be a lot more difficult to fess up to your folks if you turn out to be pregnant (I doubt that you are, but this is a good time to be taking the guesswork out of things and knowing once and for all).
One thing I'm going to suggest to you, whether you take a home pregnancy test or not, or wait until your period or whatever, is to take this as a lesson and a warning. Consider it a bullet that you and your boyfriend have dodged and do your best to never get yourself into this situation ever again. As for your saying that your boyfriend was not responsible, that is incorrect. It takes two to tango; you would both be equally responsible. But the good news is, you can learn from this experience.