Oh she is just a little tyke and many people believe 8 weeks is two young for a kitten that little to be away from her mother. The optimum time (depending on breed sometimes) is 12 weeks - that extra time lets mom show them how to do things like eating, purring, hunting, hiding, going to the litter box.
I once had to hand rear four orphans and teaching to use the litter box was not as easy as getting them to eat.
Why did the breeder adopt her so young or the rescue center give her to you at such a young age - or are you fostering her? If the latter, maybe someone can assist you there.
Usually, you should be able to show her the litter box and scratch her little paws in it and she will know that is her place to go. She may also have been scared. Also make sure she knows where the box is and since she is so very little, make sure she can get in and out and knows where it is. If it is too big find something smaller - like a cake pan even.
I should add that when you are washing your sheets, do not anything like bleach since it has amonia and she will think that may be urine again and she will go there again. You may have to keep her out of your bed for awhile. As soon as you see her sniffing around, get her to her box!!
The not in your bed part may be hard - I sleep with my kitties and cannot imagine not having them there - tho my Siamese did pee the bed once, lol (But it was my ffault - she did meow and tell me what she needed, I just did not realize it, sighhh!)