How to tell the difference???

Ok so, I know this sounds like a weird question but. How do you know the difference between a light contraction or the baby balling up??

I’m 21 weeks pregnant and I’m suppose to be watching closely because my doctor thinks I might possibly be having contractions. She thinks they are like BH but if they have a pattern then I’m to go to L&D but I’ve noticed off and on through the day that my uterus feels like it starts to harden up for a few seconds then it relaxes but how do I know if it’s contractions and not just the baby balling up??