Just a quick biology lesson! A women has a cycle that lasts on average 28 days, a cycle is the 1st day of her period to the last day before her next period! So taking a cycle at 28 days this means she should - on average - ovulate at 14 days after (IF the cycle is longer, 29,30,31 days) then she would be fertile 14 days before the next period is due! So at that time the ovary releases the egg that goes through the fallopian tubes - eggs last about 12 hours before they die off! So at that time 1 or more sperm have had to make it up to the fallopian tube, to try and burrow through the outer lining of the ovum (egg), as soon as this happens, the conceived egg continues travelling down the fallopian tube and comes into the uterus and then burrows itself into the lining of the uterus - where it stays and grows!
NOW on the other end - sperm - the penis is designed to fit snuggly inside the vagina, the reason for this is, the vagina has it's own lubrication which is to stop any foreign objects (bacteria, sperm, dust, talc etc) from getting up to the mother ship - the Cervix! There IS a chance that sperm can make it through the vaginal substance - but these don't happen very often! So the closer to the cervix the better, and the more force the better - thus the penis ejaculating! The cervix is a harder thicker gel covered "door" and she isn't letting ANYTHING inferior through her door! So the healthiest sperm can only make it through! then once through - which starting out at about 1 million in the ejaculate would now be about 100 or more! So the sperm swim through the uterus and 1 or 2 will go up the tube - IF an egg is there! And then the sperm that made it will try and try to make it through the outer shell! And as soon as they do, the shell becomes virtually non penetratable - except for twins, triplets etc. Also female sperm are the strongest - thus cooks, welders, bakers, etc. having more girls then boys (as the heat kills off sperm) and as soon as sperm hit the air or are released they start dying off by the thousands! Thus the saying "1 in a million" (I don't know if that is about sperm, but pretty ironic!!! LOL)
So just think about what sperm need to do, and to have a pregnancy takes perfect timing! BUT and this is the big one! Ovulation CAN happen at any time - usually from stress, illness, hormones, infection etc. So thus women getting pregnant right before their periods!
So infront of this screen, say after me "I will NEVER EVER EVER have sex unprotected, I WILL use a condom EVERYTIME, I will not "try it", and I WILL ejaculate into the condom when I pull out! I do not want to get anyone pregnant, I don't want a disease - that might be incurable, I want to be mature about the act of sex" Say it loud so your parents can hear!!!! ;-)
So hopefully this answers your questions, and I am sure she will have her period soon! As I said DON'T stress or you might hold it up!