Im 32wks and having the same thing. Except mine happens whether up on my feet or not. I could be totally relaxing and still be in pain with the pressure. Its bad in the back especially.
It could be activity induced stretching. As long as it goes away when you are resting I wouldn't worry too much. If it becomes constant though I would call. Just try to rest and take it easy.
It does it mostly when I'm active and doing things and it gets better when I'm relaxing so I'm thinking its nothing, I have a appt on Monday so I will bring it up then. Thanks for adviice I almost did call lol
if worried at all call your dr and talk to them or the hospital but ive had this off and on for the past month.