I haven't been doing anything different.
If you have gone through or are going through a difficult time that could be a reason. Another factor might be that you are going through a lot of changes - like upgrading a relationship or moving or graduating. It may also be that your body is trying to tell you something - if so it might be good to look at other changes like suppressed hunger or a general feeling of anxiety. If this persists you might look to either changing your habits or forming new ones. Sometimes the simpliest issue is that of age - alot of people find that sleep becomes somewhat more difficult with age.
You probably have a lot on your mind or you're trying too hard to fall asleep