18 month old hitting

My son has always been the sweetest baby. He never hit or bit with the purpose of hurting anyone. Lately when I tell him no, he gets mad and hits me. I have no idea where this came from. Obviously it doesn’t hurt, but I can’t have him doing that. I don’t know how to get him to realize that is wrong? I’m not into spanking, and I’ve noticed that slapping his hands just makes it worse. He’s started slapping his own hands when he’s told no and I hate that 😔 I feel like he’s too little to understand a time out. What do I do??