Working out

Anybody else experience the baby "balling up" while doing cardio? I'm 23 weeks 4 days, and I'm not going as intense in the gym as I did before I got pregnant, but I try to maintain a routine. Every time I feel my heart rate escalate I feel her ball up, almost like a contraction and I have to stop. Why? And what cardio would you recommend so she will chill out lol