what's the first sign of a miscarriage?

I'm exactly five weeks today. My first doctors appointment is not until march 15 (3 weeks from now) I've been having some mild cramping like period cramps. It comes and goes and I've been tired. I've been told by some people it's normal uterine stretches and some (including my mother and sister) are freaking out because they've never had cramps at all. What's usually the first sign? I haven't had any bleeding at all but they're scaring me a little. Please send reassurance. Also, this is my first pregnancy.