In my head or preterm labor?

I'm 35 weeks. This past week I've been popping like crazy lol I don't usually poop I know it's bad but not the point of this post. Anyways now I'm constipated again but I feel like I have to go bathroom.

I stood up from the couch and I got so much tightening as I walked which seems strange because when I was sitting I couldn't really feel anything other than the baby move . Just curious as to why I feel tightening once I get up and active. I wasn't sitting down for along time either

I get checked for contractions tomorrow anyways