Contractions or what?

What does a contraction feel like? I’m not sure if I’m having contractions or not. My back keeps tensing up and it feels like it is super tight and like stuck. My abdomen just feels like a mild period cramp. It is my back that is so bad. It will release in about 45 seconds to a minute and are coming every 20 to 30 minutes. I’m 30 weeks. Not sure if I should go to L&D; or can wait until tomorrow morning when I have my prenatal appointment?