I need advice!

We brought our daughter home last Saturday. So it's been over a week since she has been home. Well, our son (who is 2.5 and potty trained) won't stop pooping on himself and peeing in the floor. IDK what to do to stop him. Me and his father took him multiple times to the potty last night because he was straining, and five min after the last trip, he pooped in his pants. We're at a loss on how to stop this. He knows better. And was doing very good, until the baby came home. And were not partial, but he still gets disciplined just like if she weren't even here