Though I can't add any better advice than you've already gotten, Wendy, I just want to say that I understand exactly how you feel. My son took off on the Camino alone from Roncesvalles when he was 19, and even though I was a camino veteran and should have known better, I was worried when a week went by without hearing from him. He was of course totally surprised when he finally did contact us to learn that we had been worried. His response was something along the lines of - whatever were you worried about?!
August 14 was a mere 4 days ago. She wrote on Wednesday, and internet access is harder to come by on the weekends (as libraries and casas de cultura are closed), and as everyone has said, the most likely explanation is that she has not had time or the opportunity to write again. The Camino can be an intense and full experience, and even though people wonder how in the world the time passes when all you do in a day is walk 8 hours, the days fly by and fill up quickly.
From one mother to another, you really shouldn't worry. I knew when my son was on the Camino that he was in one of the world's safest and most protective environments. But that still didn't keep me from worrying -- so I guess the advice I've given is advice that no mother can follow! Buen camino, Laurie