BH contractions vs baby stretching

Okay this question seems so random and silly but I am a first time mom and experiencing everything For the first time. How can I tell the difference between a BH and baby stretching and pushing against me making everything hard and tight. I started having what I thought was BH at 19 weeks. I'm 23 weeks now. My doctor told me at 21 weeks what I'm feeling could be my baby stretching and pushing against me. Now I'm confused to what I'm feeling.