Hi there and welcome to TB!
You've certainly come to the right place for answers and we're glad to have you with us!
It sounds like you're taking things a bit too fast for your little one--budgies take a few weeks to fully "settle in" to their new environment. Taking her outside is really great, since it gives them vital UV and Vitamin D, but I think it was definitely too soon to take her out and she probably just got a little freaked out.
Make sure she feels calm; don't let her out for a while and talk to her reassuringly while in her cage, play soft music, work on getting to know her a little bit. Don't try to touch her or take her out for a week or two since budgies can get really stressed out by things like this, they become submissive. You can cover her cage partially with a light sheet to make her feel safer, and never leave her in a silent room, since that indicates danger for budgies.
Her ruffling her feathers is normal behavior--she's just "shaking off" and as long as she isn't constantly "fluffy" (which could indicate illness), I'm sure she's perfectly fine. Also, her "scratching" could just be that she wants to comb out her feathers while preening! Also, she could just be moulting, and have new feathers itching as they grow in, do you know her age? No, from being outside she didn't get a disease from the other birds, don't worry
If she's chirping really loudly, she is probably "flock calling". This is her way of checking out who's in the area, and she maybe is looking for her friends from the pet store. When she does this, just talk to her reassuringly and let her know you're her friend.
After it seems like she's calm in her new home, you can begin the taming process. Don't try and touch her or grab her before she feels comfortable, even if she seems comfortable at first. Just rest your hand in her cage, talking to her in a soft voice, and once she's fine with that, you can move your hand closer, then offer her millet from your hand, etc. Remember: This will not happen in a day! Only move to the next stage if she's ready.
I hope this helped and I can't wait to hear all about your little one. We would love to see pictures when you can get them and we're always happy to answer any other questions you may have after looking through the forum
EDIT: I may have repeated some things, didn't see Cathy's answer! She's absolutely right