It could be depression or it could be the absence of not taking care of your neighbor's child. When you care about someone and you can't see them anymore it is almost like someone dieing. There is a grief process I think. Only a doctor can diagnose depression. For a lot of people depression is something that last for weeks, months or even years. It sounds like you are sad a lot and might be depressed. Most colleges have a counseling department that may be able to help you. You doctor may also be able to help.

I hope you start feeling better soon.