When to worry about cramps?

I had a missed miscarriage last August and I'm currently about 7 weeks. Last time I didn't have cramps until the bleeding started. The past couple days I've had cramping lasting a few hours. I know some cramping is normal, but does anyone know how to tell the difference between normal cramping and cramping that could be a sign of miscarriage?I see my doctor next Thursday and I'm terrified of having the same experience of seeing an underdeveloped fetus. I guess I'm trying to prepare myself mentally if this is going to be another miscarriage. Any advice is appreciated!