It's not super-likely but it's also not impossible. If you are really, really worried, you can contact her doctor (if you're in the United States and some other countries) and get her a morning after pill. Not so easy and not so cheap. I wouldn't recommend it in this instance as I think the chances are small (although not zero).
Here's what to do:
- Stop fingering her after you masturbate. Instead, wash your hands with soap and water - or take care of her first. Or put on a condom when you masturbate with her. Basically, keep the possibility of sperm getting into her vagina to a minimum.
- Wait for her to get her period. If she does, it's about a 99% certainty she is not pregnant. While some pregnant women do have bleeding, a period is usually an indicator that she's not.
- If she does not, then wait two weeks after her period is expected. Why two weeks? Because stress (including her stress of hoping she's not pregnant) can delay a period. Annoying, but true. So give it two weeks.
- Then if she still has not gotten her period, go to the drug store and get an over the counter pregnancy test. She should follow its instructions to the letter. Since nearly all of them require that they not be taken until two weeks after a period is expected, you can see why I told you to do step #3.
- If she is pregnant, then I do hope you'll do the right thing and work with her through what will surely be a difficult time, regardless of whatever you choose together, as a couple.
And, finally, buy condoms and start regularly using them. If you are going this far, then penetrative sex is going to happen, sooner or later. Be prepared for that eventuality.
As for your girlfriend, she should seriously consider her own form of birth control as well, because condoms can break. Again, this is because you're probably going to be moving onto penetrative sex before the summer is over, at this rate. Be prepared so you won't have these worries anymore.