This behavior toward other people (and in certain situations other dogs) is protection behavior that both male and female dogs can get into. And yes, it's because she's becoming more mature. Puppies and immature dogs tend to be loving and playful with everyone. When she does this you have to immediately correct her or it's only going to get worse. Her changes as you described in interacting with other dogs is dominance behavior, and that also needs correcting. Dogs are always going to arrange themselves in a hierarchy of who is dominant over who, but fighting behavior is excessive, so you need to teach her what is acceptable behavior with other dogs and what isn't (this is stuff a mommy dog would teach their offspring, and as people we need to take the place of the mommy dog and correct them the same way a mommy dog would). Because dogs can only understand dog-like behavior you have to adjust your methods of training to what the dog can understand... which pretty much means you have to deal with your dog like a dog.
You have to be the authority figure in the person/dog relationship, and ALL the time. Dogs - unlike people - won't follow a leader they consider to be an unstable leader. To a dog the leader is ALWAYS in control... leaders don't freak out, they don't temporarily delegate leadership responsibility, etc.
Hate to tell you, but your dog is not respecting you as the leader in your relationship, and this may be because of small things that you have no idea of but mean a LOT to a dog. The leader ALWAYS exits and enters first. A leader ALWAYS walks ahead of the pack (you're part of the "pack" to your dog). The dog can walk beside you, but if their head is getting ahead of you, you are no longer leading according to a dog. The leader ALWAYS eats the meal first (this only matters if you and the dog share the same meal time). A dog climbing on top of you is a display of dominance over you. They can rest their head on you or throw a leg on you, or manage to get on top of you during play, but if they actually put most or all of their body onto top of you when you aren't having a wrestling match, it's dominant behavior. I could go on and on about all the little seemingly insignificant things that mean something to a dog.
Good news is, dogs are born followers... they don't really want to be the leader but will always step into the role when no one else in their "pack" does. As long as your dog believes you are an acceptable leader they'll be quite happy with that and leave you to it with their blessing.
I really recommend The Dog Whisperer. His methods are the same ones learned dog people have known and practiced for a long time, but because of his show, you can get a lot of this info when it airs or from videos at the National Geographic website often and for free. It's also instructive to see how he teaches people to handle their own dogs... that's more important than watching him handle them himself because you can learn a lot more in seeing what works, what doesn't and why through the untrained people he helps.
Here's the page for National Geographic's full episodes...
http://channel.nationalgeographic.com/channel/videos/player.html?channel=60850Choose "Dog Whisperer" from the list to view different episodes of the show. You can start by searching for the ones that may address your specific problems, but I'd recommend watching all of them because all of them will show you over and over that the same methods are applied in many different situations for many different kinds of dogs and their many different owners. I think there's something beneficial in every episode that's helpful.