This is natural because the newborn is now “sharing” your child’s most valued resource-“YOU”. Your child’s world revolves around you and having to share you with someone is painful for her. Mothers have to give the newborn a lot of time and attention (spent bathing, feeding, changing, etc) and the older child begins to feel left out and starts complaining.
Make a special time for your older child so that she knows you care. Do her favorite activities (the ones you did with her before your baby came along). This will make her feel wanted and secure.
Show your older child her baby albums and talk about the things she did when she was a newborn. Talk about the little things that she did and the special things you did for her. This will help her understand that babies need special care.
And then involve her in helping with small chores with the baby-like handing over things, singing songs to a cranky baby, etc. And tell her how much you appreciate her help.