Newborn's legs straightening when carried

Hi anyone has encountered situations where your baby straightens his or her legs when being carried? My one month old baby tends to do this occasionally. I read online that if baby does this when being picked up then it can be a sign of something serious. However, my baby doesn't do this when picked up, but more so when being put down. Feeling worried now. #pleasehelp #firstbaby #advicepls #1stimemom