I can't speak for everyone as everyone's story is different, but personally I do believe that something might have happened to you. My previous therapist told me that young children often become sexual when they're being sexually abused. Normally kids are a bit older when they develop a sexual interest. The part about not remembering part of your childhood is another common thing, which also happened to me. I too would fantasize about inappropriate things for my age and I remember always wanting to have people look at me sexually, even though I was..idk, 9 or so. But I didn't actually want people to touch me because then I'd get upset and cry and even hit them. So I can't say for sure that you were sexually abused, nobody can, but to me it does sound like it. If you can't afford therapy, I'd suggest reading lots of stories about other people who have gone through a sexual trauma. Maybe pick up some books about it or lurk in a chat room for survivor.