could it be normal or another cause
It's actually not uncommon to find yound kids, even toddlers or babies doing that. At their age it's not even registered to them as sexual behavior, simply something that can tickle or feel slightly good. Just like with a girl, it wouldn't be unusual to see a toddler boy playing with his penis and giggling, though he won't get an erection or ejaculate like later years during and after puberty, it still gives off a slightly pleasurable sensation. It's just a habit that might even last through the first few years of her life, after that she should grow out of it... but then you get to deal with it again once puberty hits.
The answer that guest posted before me definitely does cover the most part of what these types of behavior in young kids mean - they don't connect it with anything sexual, obviously, because they are too young, but simply at one point they find that their genitals can be pleasurable. Besides the obvious problems that most parents are not even familiar with these situations, they might overreact and add to the problem even more, when in fact, all that they can do at that point is to slowly and relative to the kid's age - try to teach it about privacy. But, from what I've seen in my experience, kids are more prone to finding self-pleasure when they are not getting enough affection from their environment (ask any foster-parent or a person that has worked in social services). SO, my advice is - turn the kid's attention to some other activity and give her all the affection you can - she will overgrow it (well, not until puberty hits).