Is it Autism?

My three month old started smiling big and cooing at 2 months, and she had good eye contact from the beginning. But at 3 months, she suddenly avoids all eye contact...doing everything possible to not even look at my or anyone's face. She stopped smiling. She only coos at her toys now. She's obsessed with lights and toys. She won't look at anyone in the eyes or smile anymore. She also tries to break out of the cradle hold position by crying and arching her back. She just really doesn't want to look at me. It makes me sad and worried.

Does anyone who has or knows of an Autistic kid experience this?

The doctor just said, "Autism is diagnosed at 2 years." Well, there are signs and milestones and regressions to look out for, right?

Edit:

I'm serious, I get more worried everyday. Her sister wasn't like this at all.

She absolutely never looks at me or acknowledges my presence. She doesn't need me for comfort and is totally happy if left alone by herself.

I'm really sad and even angry at the situation (how did this happen???)