my gf and i just have sex when her period is over...if it possible that she's pergnant?

Sure it's possible.  A woman can get pregnant if you have sex during any time of her cycle.  It is less likely a some times during her cycle, but the sperm lasts up to 3 to 5 days in her cervix and she can send an egg at any time really.  Some woman are like clockwork, but there can be irregularities.  I would suggest you where a condom.  What would you prefer, condoms or diapers?