Most people have new bettas or their betta has a new tank. Whatever new is to your betta fish will be a little scary to them. Think about it this way: At school your teacher says you have to talk to a person you don't know and you don't know if the person is mean or not. It is the same to bettas: The new tank your betta has may be stressful to your fish. It can also be scary! Your betta has to explore the new places the tank has. Then after 2 days or 1 week your betta will start to get used to the places. When they are getting used to the new tank, your betta might not want to eat. This is OK. Fast him/her for a few days (it will not hurt them because in the wild bettas don't eat everyday). After a little while your betta will want to eat. Just let him/her take their time.In no time your betta will act the way they were!!
I hope this thread helps!