This happened to me as well. My son was 22 months when he was weaned..and he was fine for about a month, but after a bit he just started to stick his hands down my shirt. It didn't really bother me because I really missed our nursing relationship!! (I had to wean suddenly)
then it became his new comfort. He needed it to sleep, and he needed it when he got hurt. His hands were always down my shirt. I didn't mind because I wanted him to have a way of comfort, because I felt like I took his only comfort away. I was actually glad that he had found another way to comfort himself through me. I guess it's different from your situation just because he was so much younger. But it went on for about 6-8 months.. Then I decided it was time to end it. He still at 3 wants to put his hands down there sometimes, but we laugh and say can't touch Mommies boobies. Lol. It's more of a game now. I realized that there are so many ways that I can comfort him. He is such a cuddle bug and he loves it when I just hold him.