In Act 4, Scene 3, just after Juliet and Nurse finish picking out her wedding clothes, Juliet excuses the nurse, sending her away by saying:
I pray thee leave me to myself to-night,
For I have need of many orisons[prayers]
To move the heavens to smile upon my state,
Which, well thou knowest, is cross and full of sin. (IV.iii.2-5)
The reasons why she says this are two-fold. The main reason is that, per Friar Laurence's instructions, she needs to dismiss her nurse so she can drink the potion in private. That way everyone in Juliet's household will believe her death is legitimate so that her and Friar Laurence's plan can be effectively carried through. We see Friar Laurence give her the warning in the first scene of Act 4, "To-morrow night look that thou lie alone; / Let not the nurse lie with thee in thy chamber" (IV.i.92-93).
However, there is an ounce of truth in what Juliet says to get her nurse out of her room, which leads us to the second reason why she speaks of sin in these lines. Juliet knows perfectly well that according to her religion it would be considered a sin to go against her father's will. She has gone against her father's will in two ways now: first by marrying Romeo in secret and secondly by refusing to marry Paris. Marrying in secret is not in and of itself a sin as it was perfectly legal and holy under the Catholic Church for a priest to marry people Romeo's and Juliet's ages without parental consent. However, in spirit, marrying a Montague certainly does transgress against her father's will, just like refusing to marry Paris transgresses against her father. This is why Juliet says to Nurse that Juliet's life is, as Nurse knows, "cross and full of sin" (IV.iii.5). But regardless of the truth behind Juliet's fib to clear Nurse out of her room, the sin in her life is of no importance to her. The only thing that is important to her is that she follow through with Friar Laurence's plan so that she can be reunited with Romeo and escape having to marry Paris.