Hi Nicole, welcome to PC!

I know what you mean about being depressed even when things are going well. When I go home after spending months away at school, I sometimes feel low even though I should be glad, because I'm always so homesick when I'm away. My friends and family are pretty good about it; they know that I'm happy to be with them and they know that my moods don't always reflect how I actually feel. They're patient about it, which means that if I go to see my best friend we'll just sit and chat or watch tv if that's all I'm up for, and she understands that I'm glad to be spending time with her even if I can't always show it. Have you told your mom how you're feeling? I agree with leacon -- your Mom may have noticed there's something different about you, even if she can't put her finger on what it is. It might be a relief for you to sit her down and tell her that while you're happy to be home with her again, you're not feeling quite "right" just now. Being open about how you're feeling can be scary, but I think you'll find that it's actually a relief when you don't have to pretend any more, even if it's just around one person.
Take care.