Puppy won't stop barking at other dogs?

I have a 5 month old Border Collie puppy named Panda. She is a really good dog, fast learner, and she exhibits very controlled behaviour. She doesn't bark at strangers but lately she has been barking loads at other dogs. I think she just wants to meet them, but everybody dogs are really mean. She is a really loud, ferocious barker and it sounds meaner than it is. How do I fix this?