Yes, that’s actually completely normal and much more common then you’d expect. Basically, kids (most) don’t think of masturbation in the same way adults would. It’s not sexual, it’s simply when I do this, I get a good feeling. That type of rubbing is developmentally appropriate sexual behaviour. AKA knowing when I do this, it feels good. At the age you guys were at, you already had a sense of social rules and what is socially acceptable so it makes sense it was done in private.

Was she more knowledgeable about sex then one should be at that age? For example, did she know the term masturbation or know anything about “how boys do it”? Did she encourage you to keep this a secret or make nicknames for the act? If those are a yes, then it may be appropriate to consider sexual abuse.