My Puppy suddenly becomes very aggressive. Need help.

I got a collie/cocker girl pup. I have had her for about 7 weeks, she is 15 weeks olds and she is a friendly pup and smart but she was always a bit nippy. I was able to stop her at home by putting her in timeout, saying no, but in the last week she would properly attack me for reasons I wasn't sure.

At first I thought she might be under stimulated, then over, then I was careful with certain triggers, toys, people, dogs. But then yesterday, I was walking with her fine, and for no real reason I could think of she properly went for me, it was quite scary, growling and jumping to bite me. The only way I stopped her was because of the lead and I was lucky she stopped when she did because my arm couldn't hold her back for much longer. She got me 3 times. The only thing I thought it might be was she basically didn't want to walk. I have been walking her everyday up to 4 times a day before then. She is half collie so I know she needs a lot of exercise.

Today I made sure for her to stay calm, the first walk was fine but I felt at one moment she was about to go for me but didn't. The second walk, she went for me again for no reason expect she just didn't want to walk on the lead. She rather play in the garden. She went for me, held her back, she calmed down after 30 seconds of going mad, walked, same thing, walked, same thing and she only stopped once I turned around and went back home. I feel like that is really bad especially for a pup that's only 15 weeks, not even a teenager. No triggers, calm all day, simply didn't want to walk.

Any advice anyone? I'm worried that she won't grow out of it. Like if it was a child or my elderly parents she could do damage, my parents won't even take her now. When she's big I wouldn't be able to stop her, she's already quite strong and a really high jump, she nearly got my neck already. I had a cocker and a dachshund before and had nothing nearly like this.

It's sad because I love her, and have spent so much time the last 2 months with her, I feel like I haven't done anything wrong, im not even working right now everything has just been her. I give her own freedom in my big garden also but had to limit that because that was also a trigger. I'm not constantly with her at the same time.

Anyone have any advice or similar experiences? If you have any questions just ask.

I'm waiting a reply from the breeder, there was 8 other puppies. I wonder if any of the rest of them are similar.