lying on back

Hi I'm on to my second child, my first had terrible silent reflux so I'm not really sure what is 'normal' behaviour for a healthy new born. Can I have some advice please. Can most babies be put down on the floor for a few minutes to lie on their backs happily? So far after 5 weeks my wee girl is still uncomfortable being put down for a kick or to play. I have yet to find a position where she will be happy to stay for a few minutes without fussing/crying. Just trying to decide if she's ok.