Social Anxiety Got Triggered Today

Not feeling too great emotionally today. My Social Anxiety Disorder got triggered today while I was at my doctor's office. I was checking the time and my text messages on my cellphone while waiting in the reception area, when I heard a voice saying "hello, hello". I didn't immediately respond as I did not know who the speaker (an old lady in a wheelchair) was talking to (I was looking down at my cellphone, so I didn't see if she was on a cellphone talking to someone, plus she could have been talking to someone at the front desk, for all I knew). But it turned out she was talking to me, and she got mad at me and acted like I was being rude to her by not immediately responding. I tried to explain that I didn't know she was talking to me, and mentioned the things above (e.g. that she could have been on her phone or talking to someone at the front desk), but that just seemed to make her even more annoyed with me, and she continued to act like I was being rude to her. I got so upset that I had a social anxiety panic attack, and it took talking to my doctor about it and having some water to drink to calm me down. I would never just deliberately not respond to someone's "hello". I'm not used to random strangers chatting with me: even within my own immediate family I often fade into the background and am ignored. But I was not deliberately ignoring her. She totally could have been talking to someone else.