How would helping your girlfriend get birth control be going behind her back? Your girlfriend would have to know about that, since she'd be the one taking the pill, right?
Planned parenthood gives out Plan B to anyone of any age if you have one near you. Also, if you have any friends that are over 18, they can get it from any pharmacy. You don't need a prescription to get it at a pharmacy, but you do have to be 18.
You definitely have to talk to her. To have sex, you should trust each other enough to be able to talk about things like this when they happen. It is not good to hide them.
As for your questions about pregnancy tests, you shouldn't take one until 14 days after you had the unprotected sex if you want an accurate result.
If you do get plan B, get it ASAP because the longer you wait, the less effective it is.
Good luck