Help if you can .. thanks

Okay so I’m approaching 21 weeks now (yayyy) but I’ve been feeling this really hard pressure at the bottom of my stomach sometimes (especially if i’ve been on my feet go long or if I’m tired) and it’s like a sit your ass down kind of pressure I’m not sure what it is but he also moves when the pressure stops!! Anybody else had this or know what it is? Any feedback is helpful thanks so much