Hi, it's hard to say what it means, but unless you have often had a desire to have a sexual experience with a child, or you fantasize about such things, I would say you are not a pedophile. Everyone has random unusual impulses every now and then; certainly 12-year-olds do. It does not sound like you are saying you desire to press against small children (or ever have, other than that time you described). If you were a pedophile, you would know that is what sexually turns you on, which it sounds like it does not.
Young kids often press against a doll, blanket or stuffed animal with their genitals. It doesn't mean they have a doll fetish or a blanket fetish, it means they have discovered that their genital area has a lot of nerves that tingle and they have found something to press on.
The only part of your story that (in your shoes) I'd wonder about a bit is that you chose a living person instead of an object to press against. That makes it seem like, at 12/13, you didn't see a difference between a 1/2 year old as an autonomous person with the right to the integrity of her own body, and anything else you might press on. Any ideas about why not? I wouldn't worry about it too much -- after all, kids hit, shove, tickle and hug each other all the time and are not troubled by invading each other's space. But the part I'd think about is why your cousin seemed so available, or possibly if you yourself ever got invaded in such a way when small.
If you have access to a counselor and it is troubling you a lot, you might get a lot of benefit from seeing one.
Hello!
Remember to just breath. Sometimes just thinking you are something can make you feel like you are. Can you remember having any sexual desires involving a young child recently? Or has that been the only one?
-Norden77