6 month old tenses up

Our daughter does this thing where she will bring her hands to her chest, lift her legs and sort of tense up. She makes little grunting noises. She’s aware the entire time and there are no jerky movements. If we pick her up or stand her up she’s fine. If she has a toy in her hand she will press it against her chest/face very hard until we take it away. She’s on medication for reflux but this still happens. I can’t seem to find anything about it online. Does anyone else’s baby do this? Or does anyone have any idea what it could be? Appreciate any info anyone can provide!