Has she always been shy or has her behaviour suddenly changed or got worse recently?
She has now hit her teens and being a mum with two teenagers i can say that it is normal, to an extent, for them to withdraw. Teens are famous for just hibernating in their rooms, spending their time on the computer and listening to music, it is a phase. Their hormones are all over the place and their bodies are developing etc so it is a very confusing time for them, trying to find out who they are. What is she like when at home? Is she withdrawn with the family too?
I would approach the school and have a chat with the teachers just to make sure there isnt any bullying going on that is affecting her confidence. How are her grades? How is her mood in general, is she always sad, bad mood, irritable? What does she say when you ask her questions?
Personally until you make further investigations i would hold off taking her to therapy. That is a massive step to take for a child.
Good point Ramesh. I was worried about my daughter too when she was around that age. I pushed her a little to get involved in some activities and she found out that she really loved soccar. I think being good at that really built up her confidence.
She should have friends at her age, or it becomes very lonely for her. If she isn't involved in any extra curricular activities, have her get involved in some. This is a great way for her to meet others with the same interests, and helps to bring her out of her shell. Shy is one thing, but this appears to go a little beyond just being shy. Talk to her teachers and find out how she interacts with her peers, this can be a big help to you. Get to know the parents of her classmates, this is a great way to get the girls together and you can invite one at a time to do things with you and your daughter, or drop the two of them off at a movie, mall etc. Once she becomes comfortable doing things with one at a time, then bump it up to her inviting 2 or 3 over to the house, or to go to the mall or a movie, etc. Extra curricular activities is a great way for you to meet the parents of other girls her age and makes it easier for the girls to get to know one another. If she isn't receptive to this, then I would get her into therapy to help with her shyness. The more involved she is in activities that force her to interact with others, the more she will be able to open up. It will take some work and prodding on your part, but I feel she can overcome this. I know how concerned you are as a mother, but she'll be okay with some help. I wish you and your daughter all the best!
If she doesn't open up to you and tell you what's on her mind you can take her to see a therapist that helps children. It doesn't have to be something serious because some children are naturally shy and may need to get involved in activities so that they'll open up more but you're her mother so you should know if something is really abnormal. It wouldn't hurt to go to a therapist for a consultation though. Good Luck :).