7 month old suddenly become reactive

I have a 7 month old female dachshund who was the most sociable little dog until these last few weeks. I took her to puppy classes when she was four months old and she did well.
I take her to coffee shops and she is good as gold indoors. She pulls on her lead and barks at everyone she sees when outside.
She showed fear, backing away when a neighbour came to speak to us yesterday. Previously she would have been wagging her tail and looking for attention.
I know that nothing has happened to bring about this change and she is probably adolescent and due her first season.
Any help gratefully received.