Toddler hitting

Y’all. I need help. My son is legit the sweetest kid on earth but when he’s upset about being told no (which isn’t rare...) he hits. Immediately. He’s slapped me, pulled my hair, hit me. No matter what I do, it doesn’t stop him. I’ve smacked his hand and told him no. He just hits me back. I pop his butt. He’ll stop right then but hits me again later. If I just tell him no sternly, he just keeps hitting. I’m at wits end. I’m 26 weeks pregnant and constantly worried about my kid hitting me and getting too rough. I don’t know where he got this from. His father and I don’t hit. He’s not in daycare. It just came out of nowhere. He also kicks when I try to change his diaper. What else can I do?