May be overreacting

I’ve been posting a lot lately... I feel like everything my son I’m questioning if it’s “normal”. I noticed after his 6 month shots and flu shot earlier this week that he’s started this new thing... he will be laying flat and like arch his back and make his chin touch his chest and either open his mouth wide while his eyes are wide or close his mouth with his tongue sticking out. He’s never made this face before and it’s been happening a lot the past few days. The pediatrician said said he looked wonderful at his check up it just has me a little concerned. I’ll be contacting the pediatrician Monday when they open just to run it by them I was just wondering if anyone else had noticed their babies doing this. I know it could just be a new facial expression he’s learned but it does kinda concern me since he arches his back and seems to get stiff while doing it. I hope I’m just overreacting 🙏🏻