What is wrong with me?

So I'm a 17 year old female, and life is going great at the minute. My familys great, friends are great, I have a great relationship and I've just finished exams for the summer. Only thing is I constantly feel upset! I actually want to cry ALL the time, when I'm usually such a happy outgoing girl. It's got nothing to do with sleep, becase I get just the right amount of sleep everynight. I really wanted to ask because I had to walk my little brother to school this morning and when I came home to the empty house I just broke into tears, even typing this I feel like crying, this just isn't me. I'm actually quite loud so I guess I'm giving everyone a break lol. Can someone please explain why I'm having these feelings? It's driving me crazy :( Thanks x