How to know when you’re dilating or in preterm labor

Well my doctor said the baby is measuring kind a big at that everybody thinks I’m bigger than what I am. I kind of had a little pain earlier walking back w my daughter from the park . I know sometimes when you walk too much you can get a little cramping but it just kind of felt like I could barely walk and I was in pain. I kind of feel like he hardens and my intuition is telling me that he’s coming earlier I don’t know whether or not or I just may go into labor early. what are some signs