Border collie puppy aggressive behaviour?

I have a 12 week old BC puppy named Panda. She is a great dog, really smart, fast learner and and sweetie. I love her a lot. She is my kid lol. She does great at the vets, in the car and on walks. She knows the commands sit, stay, come, down, off and paw. I train her for 15 minutes, 4 times a day. She has been having some bad behaviour lately though that worries me. Well she is a bit food aggressive. I have been hand feeding her and it seeems to be helping a bit. The only thing is that when she gets a piece a food like say she finds some outside, she will growl and snarl if you try to get it. What to do about this? I make her sit and stay for all the food she gets. Also just yesterday, I was taking her to meet my cousin. When i opened the door she started barking at my cousin. Was this because she smelled a dog on her? When I told her to stop she quietly went to let my cousin pet her. Also when her dog came around the corner she was barking and growling very meanly. She seems to like other dogs, and she lives with a shih tzu. I have never seen her act so meanly to another dog. Is it because her dog is already a sassy dog to begin with and my dog did not like that? Why did she do this? I just want to nip these behaviours in the butt now while I still can. Thanks.