baby straining in high chair??

[mrsmoomoopoopoo]

Anyone else's LO strain when they are sat in their high chair? Like they are trying to poo?
Dd strains that much the veins pop out in her head and she goes bright red like she's going to pass out.
I take her out of chair immediately and she stops. When she's straining she never actually does a poo.
She goes to toilet regular so she's not constipated.

I've tried putting a cushion under her bum as maybe the chair is too hard but that doesn't help.

Anyone seen anything like this and have any suggestions to stop her straining?

Thanks

[MeLittleDuckie]

DS sometimes gets bored towards the end of feeding so strains as a little protest to tell me he wants out. Could it be that she just wants out of her chair?