First and foremost, Juliet is worried that the powerful sleeping potion given to her by Friar Laurence might not actually work. Because then she'd have to yield to her parents' wishes and marry Paris in the morning, and that's the last thing Juliet wants. But if the worst comes to the worst, Juliet is determined that it won't happen, anyway. She's already decided to stab herself to death with a knife if things don't go according to plan:
No, no. This shall forbid it. Lie thou there. (Act IV, Scene iii).
Then, after laying down the knife, she starts getting paranoid. What if Friar Laurence mixed the potion to kill her? Who knows what's really in that little vial? However, Juliet quickly comes to her senses and realizes that the Friar is a good and holy man who'd never do anything like this in a million years.
Still, Juliet continues to be worried. What happens if she wakes up before Romeo's had a chance to ride to her rescue? Won't she suffocate in that tomb with all that fetid air? And even if she lives, won't she be surrounded on all sides by death and darkness? Juliet is scared stiff that when she wakes up in the tomb she'll be able to smell the hanging odor of death and hear the ghostly screams of her ancestors. She's worried that this will drive her completely mad and make her do something crazy like pulling Tybalt's corpse out of his burial shroud or smashing in her skull with one of her ancestor's bones.