I wouldn't worry. The most likely cause is a cold virus.
There is also a small possibility she might have injured a vocal fold during a particularly loud bout of crying. If this is the case, it might take a couple of weeks to clear up. (I'm a singer, and I actually injured myself this way by singing with laryngitis once.) It gets better on its own.
If she seems OK otherwise, I would give it some time.
Dawn - Mom to : Jack 11/04 and David 5/08