Braxton Hicks?

Ever since like 15 weeks I've had uterus tightening that would cause me to have to stop and sit down and it would make my legs kinda numb, mostly after eating but I didn't think anything of it. Now I'm 25 weeks (with twins) and it happens more and takes my breath away. I noticed it a lot today while walking around the mall and Target. Is this what Braxton Hicks are or should I be concerned? (FTM) I have my next appointment next Monday so I was gonna ask then but figured maybe I need to call before then..?