3 year old acting out?

Marisa
I've been home with the new baby almost 4 weeks. My older son just turned 3 today. For the last 2 days he's been really acting out, he's been great up until now. He's also sick, so that could be it. But I'm wondering if anyone's older child had delayed acting out about a new baby, after initially being fine? Not sure what's going on with him.