Re: pre-*** on fingers/vaginal secretions
Based on your post, you "fingered" your girlfriend without entering her vaginal opening, and while she was wearing underwear. Basically, what you did was touch/rub her down there through her underwear.
If that's the case and that's all that happened, then there is NO way she could be pregnant. Just touching a woman's private areas, especially when she is wearing underwear, won't cause pregnancy. If it did, we'd have a lot more pregnant women in the world.
Just so you know, "fingering" usually means when you actually insert your finger into her actual vagina (skin to skin contact). Even doing that is very unlikely to cause pregnancy, unless you had live semen on your finger while doing it. This live semen wouldn't be just dried semen from touching your boxers. It would have to be like if you ejaculated, then inserted it inside of her.
I wouldn't worry. What I would do is education yourself a bit more about sex and how pregnancy occurs. I would education yourself on a woman's cycle too. And since it seems like you are going further with your girlfriend, education yourself about the consequences of sexual activity -- not just pregnancy, STDs too. Remember diseases can be caused by more than just sex, they can be spread by oral sex too. But I believe there has to be skin to skin contact.
If you feel comfortable what you're currently doing with your girlfriend, I would try to just stick with that, as it probably won't spread any diseases and since it's not skin to skin contact, won't cause pregnancy.
If your girlfriend does happen to be pregnant, it is not from you, if this is the only activity you have done with her. I don't want to alarm you there, just saying that in case she does turn up pregnant -- you never know when you're just posting on a message board.
One more thing, her period is probably late due to stress from the exams. Stress is a common factor for delaying the start of a period, along with changes in exercise or possibly diet or other health issues. And, it is common for teen girls to have irregular periods, as a young woman's body is still getting used to being a woman.
Oh, and you say this incident occured before her last period. If that's the case, then her having a period means she's not pregnant.
Good luck and I suggest reading and learning more about sexually activity in general.