my son did this. I realized that if I noticed him straining I could lay down with my knees up and then place him sitting against my knees. For some reason he loved to poop this way. Even now he only poops standing up. He will cry if I am holding him to stand up so he can poop.
If her poop is watery either shes having another issue or could have some sort of compaction that's stuck in there too that's causing her discomfort. I would try to get her in as soon as you can to rule out the latter.
How old is she? Mine seemed to have the most discomfort at around 4 months.
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