Braxton hicks ???

I am 23 weeks today with b/g twins. I have been feeling tightening in my tummy off and on.... But I don't think it is bh. This happens mostly during activity like walking. Even when strolling my stomach automatically tightens and gets super hard and a bit uncomfortable. It only eases off if I stop walking, so if I walk for 15 mins it happens the entire time Til I stop which is the reason why I don't think I its bh. Anyone have similar experience ? This is my first pregnancy.