my DD is shy too. I was as a child and still kind of am. I try to say it takes her a while to warm up instead of shy. But I wish there was a single word instead of shy. Saying it takes her a while to warm up or that that she's more reserved takes longer to say.
I do still say she is shy sometimes. But I really don't want to create a stigma or label her. I hate it when people say "oh don't be shy!" like it's a bad thing. Most people don't say that though. When I was a child I remember a few people saying the "cat got my tongue". My dad would joke that they hadn't taught me to talk yet.