Welcome to HCM.
Intercourse just before expected date of period, is extremely unlikely to result in pregnancy
. Though you did not have actual intercourse, your deposit of semen on her panties, could seep through depending on the material of the inner wear.
I think most likely the periods may be delayed and you may have nothing to worry.
A word of caution : your girl friend's age is too young to indulge in such activities, and see that you do not repeat such thing in future, till she is of mature age.