Hand flexing

My 5 month old sometimes has spells of flexing her right hand, like she’s squeezing something. She does it when she’s sucking on a dummy, or drinking from a bottle or sometimes when she’s concentrating. A very rude family member told me it’s a sign of autism, and now I’m freaking out! Is this just something baby’s do in development, or could she have autism? She seems to be reaching milestones, smiles and interacts with me, holds my gaze, answers to my voice etc. But I can’t stop thinking about this comment!