I like a girl but she has a boyfriend I said her I have a crush on her.I even used to text her( just like my love,my darling) she even said thanks when I used to say that to her.But suddenly after some day's she turned rude and said me to stop loving her why what's the matter.

She doesn't want you as a boyfriend, she has someone else and has no time for your smitten texts. Take the hint and stop!

The problem here is that you don't seem to understand the word NO. She has a boyfriend and she doesn't want your affections. If you keep pressing it they call that stalking. So stop right away and go on with your life.