"Oh if I wake shall I not be distraught environed with all these hideous fears and madly play with my fore fathers joints and pluck the mangled Tybalt from his shroud and in his ragewith some great kinsmans bone as with a club dash out my desperate brains"
"...when I am laid in the tomb, I wake before the time that Romeo come to redeem me? There is a fearful point. Shall I not be stifled in the vault to whose foul mouth no healthsome air breathes in, and there die strangled ere my Romeo comes?"
Juliet is afraid of being trapped in a tomb with the ghost of Tybalt.
Juliet's fear is that the potion won't work and she will be forced to marry Paris. "What if this mixture do not work at all? Shall I be married then tomorrow morning? No, no. This shall forbid it. Lie thou there."
Juliet has the fear of suffocating in the tomb before Romeo arrives to save her from the tomb.