Please tell me, I'm 30 weeks pregnant today, and for about a week now …

Please tell me, I'm 30 weeks pregnant today, and for about a week now there's been some cramping in my lower abdomen or uterus, I don't know, just something down there that feels like a cramp for a couple of seconds then lets go. My back also jerks during the cramping. The doctor says there's no uterine tone, everything is normal but I'm scared about the baby, could this be anything?