Pregnancy scare! Please help!

Probably very low. However, precum can sometimes have sperm in it. Chances are low but not zero.
http://americanpregnancy.org/getting-pregnant/can-you-get-pregnant-with-precum/

Sperm's got to get from penis to vagina and then to egg. That's a lot of traveling for a microscopic thing, particularly one that really can't get dried out. They don't breach and leap like dolphins. But you're not helping matters any by getting your penis close to her cervix via intercourse. You're bridging the gap - a lot - for something so small.

As for her dreams and her being late one day, those are most likely not going to matter (the dreams in particular mean squat). Stress in particular can make a period late - and that's rather annoying when it's stress about a possible pregnancy.

If she is late for 2 weeks (14 days), then go to a drug store and buy a pregnancy test and follow its instructions to the letter so that it has the best chance of being accurate.

That having been said, it's time to go to doctors. Yours and hers. Talk about how sex really causes pregnancy, and get to know just how to prevent it. It's also a good time to ask about STDs. You and your girlfriend may be perfectly fine, but the people you meet later might not be.

This means a two-part protocol: condoms and some form of birth control for her. These, together, will keep the chances of pregnancy down to a minimum and will help to prevent STDs as well.

And if you're underage, you can still talk to your doctor about birth control. In fact, it would be a lot better if you did. You have the right to medical privacy, just like people over 18 do. This means, unless you're telling your doctor you want to kill someone, what you say is confidential.

One last thing. Buying condoms, for either of you, should be as easy as buying a pack of gum. If you can't handle going to a drug store and purchasing condoms, then you're not ready for sex.