EDIT: the original title of this post was "is this plagiarism?? what should i do now??"

To answer the title question: this is not plagiarism. Plagiarism is presenting another author's ideas, text, etc as your own.

Since it seems from your description that you are still involved in the described project, no such thing has happened.

It seems more that your professor has taken the leadership of the project (which is not strange for a project running in his group), and taken decisions that make you feel less comfortable.

Your fear is that you will not receive (enough) credit for suggesting the original idea. I think that it is best to address this in an early stage (i.e., now) to your professor. Most likely he will assure you that you will receive credit for the original idea. Write down any verbal guarantees that he might give to you in an email to the professor. Then during the execution of the project, make sure that any senior project member is aware of any agreement between you and the professor (e.g., by forwarding the email).

Try to do this in a graceful and polite manner, and try to avoid appearing (passive) aggressive.