Juliet's internal conflict in Act IV, sc. iii is based on her fear and uncertaintity about what she's done and what she's going to do.
She is worried the vial will not work and she will awaken in the morning and be forced to marry Paris; she tucks a knife under her bed--implying she is willing to kill herself before marrying Paris.
She is conflicted that maybe Friar Laurence, who risks his reputation if he marries Juliet to Paris after he's already married her to Romeo, could have put poision into the vial in an attempt to actually kill Juliet and cover up his guilt.
She is frightened that if she awakens in the Capulet crypt earlier than Romeo arrives to take her away to Mantua, that she might go crazy being surrounded by her recently deceased cousin Tybalt and his ghost who would want revenge on Romeo. She also fears awakening too early and being surrounded by her other dead and decaying relatives. Juliet is worried she might go mad from the stench of the air in the tomb and suffocate as a result, or go mad and bash her brains in with one of her ancestors' limbs.
Juliet's internal conflict is on deciding whether or not to follow Friar Laurence's plan.