The people of a certain congressional district in Northern California elected Nancy Pelosi. Slightly over half of the House of Representatives voted her to be Speaker of the House. She has not been appointed by anyone in the Executive branch to be an ambassador.
Now, as an American Citizen, it is perfectly acceptable for Nancy to travel anywhere in the world she wants that they will let her in. Doing so claiming to be some sort of official representative of the United States ... it is not acceptable. She can talk to the Syrians about the wishes of the people of San Fransisco all she wants, as long as foriegn officials and the rest of the world understand that's all she's empowered to do.
I noted she covered her head while visiting certain areas/people. I guess she decided to wow President Bashar Assad with her gams, though. I'm no fan of yours, but hey, Nancy... they're not bad! Don't say I never said anything nice about ya.