I am quite worried about my baby. She is 3 months old now, and she does this really weird, stiff back arching thing. Often, but not exclusively, when she is upset. She is stiff enough that if I lie her down on her back I can slip my hand under the small of her back without it touching her, and generally she ends up lying slightly on her side because I can't physicallly put her down on her back. I first noticewd this when she was a few days old, but haven't mentioned it to anyone... but I do worry about WHY she is doing it?