Maybe you aren't challenged enough. Try looking a little deeper into one of your topics. Outside research of a topic can be interesting and can bring up great discussions in class. I sometimes catch up on current events and bring them up in my History Class.
You could be depressed. Or going through bad times like parents' divorcing, or a death in the family or the death of a pet or friend. If so, find someone you trust and talk to them. Or you could ask your school counselor for help.