cramps 27 weeks

Why do I have cramps so bad?? I went to L&D yesterday and got monitored but I wasn't having contractions and I'm not dialating so they sent me home. They said I'm low risk for preterm labor. I've been drinking loads of water and laying down but nothing is helping. The cramps are in my lower back and it feels like period cramps. It's almost unbearable and it kept me up most of the night last night. Is this normal?? I'm just so confused. I'm 27 weeks