How did this happen?

How did my sweet precious angel turn into this sassy, biting, talking back little girl. She  turns 17 months tomorrow and is already far into her terrible 2s. Her favorite thing to do right now is to slam her bedroom door in my face and laugh i thought that didnt come until at least the teens. A few weeks ago she was playing in the curtains on the guest bed and i didt want her to fall off the bed amd get caught in them and get hurt so i told her to stop. She ignored me the next 4 times i told her and the 5th time i told here that if she didnt stop playing in the curtains i was gonna spank her. Her responce was "well spank me then". I was on the phone with my mom and she heard her and started laughing. I think i was in shock this is my sweet little angel that cant do anything wrong and she is talking back to me. It seems her behavoir keeps getting worse. A few days ago i told her to sit dow in her chair and she promptly yells back "make me". If i dont give her my attention all the time she starts yelling my name until i answer her then she smiles and runs away or comes up and hits me with a toy. She is smart as can be ad i think that is half the problem she is too smart for her own good and she knows it. i can see her haveing lots of behavior problems when she gets in school. o well that is a long time from now and we will cross that bridge when we come to it. For now i would love to get her into gymnastics i know she would love it and i thought it would be good for her. It would give her something constructive to put all that energy into i hope. I love her so very much but she can be so trying at times. I would do anything for her though.