It's not uncommon for border collies to be fearful, especially when they are young. A lot of them have fears that (if handled incorrectly) can follow them their whole lives, like fear of loud noises or men wearing hats.
I would take her for lots of walks where she has plenty of opportunities to meet strange dogs. When she begins to react fearfully (tail down, ears down) it's important not to reinforce her fears...do not "comfort" her when she is displaying fear of another dog, simply keep walking as if nothing is wrong. Try to catch the very early signs of fear and don't let it progress to the point where she is encouraged to interact with a dog she is uncertain of. After sailing past many dogs over many days with no particular reaction from you, she will begin to understand that you don't think those other dogs are worthy of any particular response and she will begin to relax. When she is walking past other dogs without any reaction, praise her gently and then gradually work up to "meeting" other dogs. Just never try to reassure her when she's showing fear, or you'll help cement that reaction. | 
Fear nipping 
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