What could this be?

This is ligaments stretching. And it gets worse with each pregnancy. I have the same problem. Hits ya like a brick!! I wanted to cry the first time I got one asked the doctor and looked it up and it's fine. No big deal just your body making room for your munchkin! Oh, and the reason it's worse when you are standing is simple... gravity. It pulls the baby down which causes the stretching when you stand and walk. Take care and Congrats!