I have twin 1 year olds, since my girl twin could sit up, she would tense up whenever we put her in the high chair, clench her thighs, pull her whole body inwards, her face would go red and hot and then she would appear to pass out! We assumed she was pulling against the high chair and cutting off her blood supply so we changed it for a high chair that she couldn't pull her legs behind. However the tensing up carried on although she hasn't passed out since then. But it then started happening in her car seat as well. So we came to the conclusion she didn't like the position so we ditched the high chair and bought toddler table and chairs and bought a car seat that we could tilt back. Everything has been fine since then. Until the last few weeks. The twins are 20 months old, and she has started tensing up in her tilted car seat and then today she has done it just while nodding off on the couch which is new! Does anyone have any idea what it could be?