Yes, back pain is indeed a sign of pregnancy. However, the only way to be certain is if she misses her period, an then do a pregnancy test.
For future reference pulling out before full ejaculuation is not an effective form of pregnancy prevention. Your pre-ejaculation still contains sperm that are quite able to fertilize the egg, resulting in pregnancy. Too many young people believe that the pull-out method works. This is a myth!
If you and your gf do not want the responsibility of a child then you need to start using birthcontrol. The best thing would be for her to get on the pill. If this is not a possibility the next best thing would be for you to use a combination of condoms and spermacide. I say a combination of these two because condoms are not 100% effective. They do break. Just make sure the spermacide is compatible with the condom. Some spermacides can work against latex, making it weaker.
While pregnancy can happen at any time, a woman's most fertile time tends to be 11 - 17 days after the first day of her last period. On average the woman will ovulate 14 days after the first day of her last period. Sperm can live for up to three days, thus the 11 - 17 days. A woman's lest likely time to get pregnant would be during her period. Keep in mind that this is not a sure-fire thing, either. Every woman is different.
If you continue to have unprotected sex, pending both of you are fertile, then pregnancy will eventually happen.
