help please!!

my baby boy was born at 30 weeks 3 days via emergency c section due to me having a severe high blood pressure that was not going down no matter what. he is ok and responding to treatment.just that he's been straining,grunting and stretching for more than a month now.when I ask doctors the reason why he does that, they only tell me it's normal among preemies.i just want to know if other mums have been experiencing the same and if it's normal.am getting worried. thanks