C.T.
When my son was 3 and 4 (and the start of age 5 before he got teased by another boy) he loved to wear girl skirts and princess dresses. He also loved to stuff a stuffed animal down his shirt and pretend he was pregnant and going to have a baby. Perhaps your son was pretending this? My son also loved to pretend to be the girl character from cartoons and his favorite movies. He really identified with the girls - I think because they were more sweet and cute and kind. He did not hide it though - he did hit quite gleefully. He is 8 now and would be horrified if I told him he liked to do this stuff...he now is really into soccer, ski team, popular music, bmx biking, and video games. I guess keep trying to talk to him about it or see if he will let you pretend with him. Then you can gain some insight. Maybe his sister told him not to go in her room and mess with his stuff? Maybe his siblings told him that girl clothes are only for girls and teased him? Let him know you think it's just fine and ask him if he wants some of his own girl clothes to dress up in. See if he will let you play with him and maybe he will tell you what he is thinking/pretending. Don't tell him anything he is doing is wrong or make him feel bad (sounds like you already know that).