Fleas? or something else?

We just adopted Lena from a very respectable pet orphanage. Supposedly
she was cleared by the vet at the orphanage, but since she came to the
household, she's been scratching more and more - left chin, right chin,
then all around the neck (but no other places). At first I thought her
skin was irritated by the new collar that I had put on her, but even
after I removed it, she was still scratching. Now I'm all paranoid that
our other cats got fleas from her because I've also noticed some neck
scratching activity among our resident cats (although not as frequently
as Lena). We have never had experiences with fleas and don't know
where cats usually scratch when they have fleas, so do you guys think
Lena has fleas and or could there be other skin problems?