Communicating with my audhd 18 year old daughter

Hi, I'm new to this forum and have been hovering around support groups and forums for a little while now. My daughter was diagnosed with Autism and ADHD (inattentive dominant) last year at the age of 17. I'm worried that she has social anxiety and want to open up some discussion about this but don't know how. She doesn't come to me with any issues and is incredibly private and independent. If I try and ask her about anything she tends to shrug or say she doesn't know (this can be to anything, even things I know she knows the answer to). I'm hoping she's just being a teenager and wanting me to butt out but I've seen her looking uncomfortable and anxious when around others even when she's at home and I want to make things less difficult for her. Since her diagnosis she hasn't wanted any support, she's got a couple of friends but she's seeing less and less of them and staying in her room more. 

I appreciate that I am a worrier, my husband thinks she will just be ok and to leave her to it. I've had social anxiety for many years and would do anything to make things easier for her if I can. Advice on opening up any kind of dialogue would be greatly received. When she was a bit younger I  used to ask her to write me little notes when she couldn't tell me what was bothering her, she doesn't seem to want to do that anymore.