You actually have to have sex in order to get pregnant.
It's nearly impossible that she is pregnant.

Stress and being worried will delay her period. That is probably why.
If her period doesn't arrive in the next few weeks, she should see her doctor.

Not everyone has a regular, exactly on time 28 days schedule (actually, it's quite rare). It's completely normal (especially if she is still a teen/early 20s) to be off by a few days, or even weeks.

It would be a good habit for her to start writing down her periods (start, stop, length and full cycle days). I'm not sure how you measure it now, but don't rely on every 4 weeks or "it starts on the 14th of every month".

Yes, stress can definitely cause a delayed (or even missed) period. I agree with Marianne, if it doesn't show up, have her see a doctor.

No, you don't have to have sex to get pregnant. You can get pregnant any time semen comes in contact with the vagina (such as if it drips from the anus during anal sex), and also if there is semen (on hands or fingers) that comes into contact with the vaginal area there is a possibility of becoming pregnant

I agree with Lilly Ivy, have her mark when she has her periods. Just get a small calendar book to keep in her purse or wherever and just mark when her period started and when it ends.

if shes "late" all the time then like lilly ivy said its possible that she just has a really irregular cycle..i had always been that way and it didn't really bother me until i became sexually active and i starting keeping track...or..trying to anyway...i freaked out one month because i was nearly TEN days "late"...i counted the 28 days, no period...finally after 38 days I got it again...i was under alot of stress during that time but after that i went on birth control just to get my cycle under control and of course to prevent pregnancy...maybe this is something she should look into..since hers does seem to be irregular, this will help ease the worrying plus if you do finally have sex then she should be protected

the chances of your girlfriend being pregnacy are slim to none..basically as someone else said, typically the concern for pregnancy comes when you have intercourse...
i'm not sure how there would be a situation in which a guy who had semen on his hands would do anything with a woman before cleansing himself...
also..anal sex is sex