When can Braxton Hicks begin?

I’m 18w4d and I’ve been having these come and go cramps in my stomach, my stomach tightens up gets rock hard then eventually becomes more squishy. Sometimes these cramps stop when I change positions or if I change up what I’m doing. My doctor wants me to come in today to make sure everything is okay and my mom and nephews mom said these definitely sound like Braxton but my friend said at 18w I’m not able to have Braxton. Thank you for anyone who can give information