Pain/contractions while walking

I’m 16 weeks and for about a week now, I’ve been feeling increasing pain and tightness when I walk just short distances (like 1/8 mile). Today it was so strong I had to stop for a minute, and I felt a strong contraction bulge out above my belly button. It went away as soon as I sat down. I figured it was just round ligament pain at first, but could it be BH this early? Should I be concerned?

This is my second pregnancy, no complications so far... I just don’t remember feeling contractions so early on!