AWWW that's sweet, I'm glad some people take relationships seriously these days. I say; spend time with her! Try to make her laugh, smile, etc. I like it when people notice I'm sad, and they try to make me laugh and smile and it works!
This is from a girl that has gone through this...many times.
And it's fine if you want to buy her little things, it's a nice way to show you care. But don't always depend on "presents" to cheer her up. And if it works she's only using you for gifts.
Act goofy. Be romantic. Say that you love her. hug her, say that you'll be with her 'till the end. And all that stuff. <3
In order to cheer her up, you first need to identify what may be the cause of her sadness. Try to keep her engaged and be an influence by smiling and laughing but make sure the timing is right. Instead of buying for her a gift, take her out to do something that she enjoys. It's best to be patient as this may be just a passing phase and eventually she may return to her normal self.