Children 3 to 6 years old have a strong interest in both babies and sexuality that your son may be tapping in to. Seeing your interest in a newborn (I assume you oohed and aahed!) was probably a bit threatening for him, making him wonder whether he's still your baby. This almost always happens when you have your own infant but can also occur when you admire another baby. The sexual dynamic may surprise parents, prompting a big reaction that then makes your breasts even more interesting. The key here is to be matter-of-fact about setting limits without much fuss. You might simply say, "I don't like it when you touch me there." Reassuring him that he'll always be your baby and "babying" him by cuddling during a special time each day will also help. When he receives such responses, his interest should soon turn back to more age-appropriate activities.