Baby scream crying without reason

Hi Mummies These few days my 3.5mths old baby will scream cry randomly at night when it is not even her feeding time. The diaper has changed but no matter how we tried to sooth her she just cannot seemed to calm down and kept crying and screaming :( i really duno the reason except noticing that she has not been pooing these few days. We put ruyi oil on her tummy but still nothing helps. Any idea?