Quick answer: SAD is feelings. ASD is behaviors.

Pedantic answer:

Social Anxiety Disorder is nervousness, fear, agitation, and panic that is brought on by being in social situations, thinking about social situations, or just anticipating socials. People greatly worry about being judged, negatively evaluated, or rejected and it causes tremendous distress. It might ve visible (blushing, stumbling, sweating).

Other names for this include performance anxiety, public speaking anxiety, stage fright, and timidness. In the most extreme form, this could manifest as agoraphobia (fear of open spaces), enochlophobia (fear of crowds), and claustrophobia (fear of being trapped).

Autism Spectrum Disorder does include the behavior patterns you have mentioned: limited eye contact, hyper- or hypo-sensitivity to stimulation such as lights and sound. Also included are deficiencies in recognizing verbal and non-verbal communication signals from facial expressions, body language, reciprocity, humor, emotions, and gestures.

I imagine the two overlap when you are feel anxious and express autistic behavior in a social situation.