No Sex, Still Worried

My girlfriend and I don't have sex. In fact, we try to be responsible about sexual enjoyment. Generally we excercise these rules:

1. I masturbate her.
- our reasoning is that one of us may get semen on our hands if I
was done first and then her.
2. She puts her panties on
- this is so no rogue ejaculation finds it's way down there.
- also, we often leave her panties on and I masturbate her over them
3. She masturbates me.

We lay side by side, and we have very rarely excersied "dry humping" with clothes on. We do exercise this almost every day.

Also, I never fondle her actual vagina. Instead, I play with the clitoris (makes orgasming easier).

Here's the catch: a couple of days ago she complained of her nipples being extremely tender (not her breasts). Recently, she has also complained of some "lower" stomach discomfort. Her last period was about 3 weeks ago, so we have some time to become really nervous. These are her only symptoms, which could easily be explained by other things. We took a home pregnancy test, even though it is too early to, and it was negative (it helps with the reassuring, at least). I am 18 and she is 17. We are both virgins.

I would like some feedback on how possible a pregnancy could be.
Thank you.