Dear dreamer,
most women get their period every 28 days, this is considered normal although everyone is completely different, weird isn't it. If your period is coming every 2 weeks maybe you should see your doctor, although don't be alarmed this may be regular for you. If it's not your doctor will probably refer you to a gynocologyst to get some hormone treatments to get your body back in sync. Your mum is right, having sex during your period, or while you are bleeding can cause infection, and you can't get pregnant while you are bleeding anyway. It is going to be very hard for you to figure out when you are ovulating if your periods aren't regular, so you need to address this problem first. When you ovulate (or you are most fertile) your body releases an egg to be fertilised by your partner, this only happens once a month, and the egg is only there for 24 hours if that, so you need to be quick. Women usually try to catch the ovualtion period 14 days before their next period is due, there are ovulation kits you can buy to give you more accuracy in predicting your ovulation. You will find them at the supermarket or chemist next to the pregnancy tests. Below I have supplied links to some helpful information, the first is an ovulation calculator but this will probably only be helpful once you know how to predict your next period, at least once you have seen your doctor and are all sorted this should help in the future with trying to get pregnant. The second link explains what ovulation is and why you can only get pregnant on these certain days and not just any time of the month. Good luck and keep in touch.
