Periods during pregnancy can be many things.
Some women get "womb lining" growing outside the uterus - this can still be shed.
Some women still just get a monthly bleed - but not a true menstrual period.
Unless you take a case study you won't get one hard fast answer.

What normally happens to cause a period?

In a non-pregnant woman, there is a monthly cycle of female hormones which controls the release of an egg and causes the lining of the womb to thicken. If that egg is not fertilized, hormone levels fall at the end of the month, and the womb lining breaks down and is shed as a period.

What happens when a woman is pregnant?

When a woman becomes pregnant, the cycle of hormones is usually lost. Instead, hormones' levels increase to stimulate the womb and make it ready to support the developing foetus. So no womb lining is shed - and there are no periods during the pregnancy.

What could explain these rare cases of periods during pregnancy?

There are two possible explanations:

hormone levels aren't high enough. In some women, the hormone levels in pregnancy aren't high enough to prevent the lining of the womb being lost in those areas where the placenta isn't attached. So sometimes a woman will have one episode of bleeding or more in the first month or two of pregnancy (and, rarely, throughout the pregnancy).
The bleeding is due to something other than a period. For example, a pregnancy-related cause of bleeding, such as a tubal pregnancy, or another cause, such as polyps on the cervix. In these cases the bleeding isn't usually regular like a period.

Thanks for all that info! That did explain a lot...But in the first case that the article described (i think it was an article lol), the one about hormone levels not being high enough...Would the woman bleed like a period in that instance? For example, my periods generally last about 5 days, and are extremely heavy the first two days with terrible cramping during the first three days (and by "terrible" I mean I need to take prescription medication every 6 hours plus aleve every 4 hours plus a thermacare patch just to feel slight relief)...If a woman were pregnant and her hormone levels werent as high as they should be, could she expect to bleed like that??

Personally, I wouldn't think so. Plus, you would need to make sure those meds were ok to take during pregnancy else you could run the risk of birth defects.
Where it is hormone levels related it is usually only the first few months.
I was interested to see the bit where it said about the womb lining detatching, from where the placenta isn't.

The article was from about.Com - they seem to have articles on nearly everything, maybe have a look next time you are online.

If you are having your standard regular style periods, I would be reasonably sure you're not pregnant, especially if you have been keeping your clothes on.

You can get birth control in the form of pills, patch or shot which would help alleviate your worries, make sure you also use a condom of you have intercourse.

Thanks so much kia and no, we wont be having intercourse, we're both virgins and good god could you imagine how much we'd worry if we were actually sexually active?? Lol we'd never get any sleep!

No probs, any time, just ask and my opinion you can have
also about.Com is a great resource.
If I don't know and you want me to look it up i'm sure I can somewhere, if it is in one of my pregnancy books, I can scan it for you

Unlike miss.Kia <3 haha I dont know all the pregnancy scientificness hahah however I do know someone who had their period up until beinf almost 8 months pregnant she gained no weight nor anything then missed her period took a test went to a doc and found out she was 8 months she ended up (luckily) having a very healthy baby girl (6 lbs ?Oz) but yeah so I guess its possible