The truth is, you won't find an simple answer to your question on this site because there is no simple answer.
How fertile you are at different points in your cycle depends on exactly when you ovulate, and that can vary a lot from woman to woman and from cycle to cycle, especially if you've started menstruating within the last 5 years.
Unless you are charting your periods, monitoring your cervical mucus, and taking your temperature daily, you're not going to be able to tell when you are ovulating with any degree of accuracy.
Sperm can survive in the body for as many as 7 days. So if you have unprotected intercourse during your period and then ovulate early, it's perfectly possible that the sperm might still be in your body when you ovulate.
Potentially, you can get pregnant at any point in your cycle.
And just trying to guess when you might be least fertile is very, very unreliable as a way of avoiding pregnancy (and of course unprotected sex also puts you at risk of STDs).
Check out http://www.scarleteen.com/forum/Forum6/HTML/006106.html for an answer to a very similar question.