Some cats will throw up or have a big bowel movement right before delivery. If she is going to her box, she must be about ready and she is already having contractions. Just make sure she doesn't have contractions (with no result) for too long and become exhausted. If she does that, she may be having problems.
Also, know your cat's personality. If she is extremely focused on you, it may be good to sit beside the box and assist as needed. Let her try to do it all herself first, though. If she is the type that would rather have privacy and acts too nervous with you around, leave her alone, but make regular checks on her to make sure everything is ok.