She is frustrated because she lied and has got caught.
Check it out:

Leading Republican senators charged Thursday that National Security Adviser Susan Rice’s public account of the Benghazi terror attack has now “absolutely collapsed,” citing inaccuracies in her statements not only on the origin of the attack but the level of security at the U.S. compound.

The lawmakers said she is clearly “frustrated” that her story is falling apart, a day after Rice appeared to scoff at a question on whether a congressional select committee probe would reveal new evidence. “Danged if I know,” Rice said in response to the question Wednesday.

“She’s frustrated this won’t go away,”Sen. Lindsey Graham, R-S.C., told reporters at a Capitol Hill news conference along with Sen. Kelly Ayotte, R-N.H. “She’s frustrated that she appeared on national television and told a story about Benghazi that has absolutely collapsed.”