Shy or autistic?

My daughter is 19, almost 20 months. I am a SAHM and she’s had very little interaction with other kids her age just because no one I know has kids. I took her to swim classes every week for a few months last summer but that’s more interaction with the parent than other kids and did a story time group but in any situation where there is a new person or kid she will literally cling to me and look terrified the whole time.

If we are in a restaurant and there are kids she will stare at them and appear interested but the moment a kid approaches her she will freeze and just stare and have a blank look on her face.

She also didn’t start crawling till 12 months and walking till 18 months fully on her own without cursing.

She also sometimes gets excited and will open her mouth and flap her arms.

On the other side of things, when she is with my family or people she knows she is totally engaged. She had never had language trouble and at this point speaks in sentences and knows probably 80+ words she says on a daily basis. She makes full eye contact and enjoys imaginative and interactive play and games. She knows how to respond when asked what her name is and knows how to ask for what she wants.

Yet when she gets around anyone else she freezes and won’t say a word or smile. Is this just her being shy and afraid, or do you think these Are autism traits?