There are two reasons. One is most common constipation.
If your stomach is not empty, you will have no desire to eat.
If this is the reason then treat it. Take plenty of water & Metamucil one tablespoon with water every night.
The 2nd reason is lack of activity.
You should take part in games. Walk daily for 40 to 45 minutes.
Inshallah you will be all right.