Hand Flapping, Head Shaking

I’m a FTM and I have a 13 month old son that was born 3 months premature. His adjusted age is 10 months and that’s what he is developmentally. About 2 weeks ago, I noticed he started shaking his head side to side. He did it the first time on our way back from the park, now he seems to do it when he’s playing or overwhelmed. He does it several times a day for a few seconds at a time. Just about a week ago he started what I thought was waving. He flaps his left arm up and down. He doesn’t do this in response to me waving at him and does it only with his left arm, several times throughout the day, when he seems to be content and at play. His PT said she would take note of it, talk to his other therapists, and keep an eye on it. Anyone else’s kids do this? Any ideas what it is?