Painful pressure ?

When I walk anywhere past 1.5miles, I get the worse pelvic pressure pain ever. It feels like I need to poop, like an upset tummy. But its really low, like right beneath my belly. It almost feels as if the baby is using my pelvic bone to cradle his chunky butt LOL (his tummy only measured a day higher)

Today, it was really bad. I got home after walking, pooped, and ended up having to get in the cool shower because I really thought I was going to pass out!

Could it be I'm not drinking enough while walking? (I usually carry a 32oz bottle with me) or could it be something more concerning ?

I'm a FTM and 21w1d