Mayo's problems are not related to any physical injuries from his fall in Stage 3.
Procycling reports:
""I just felt a terrible lack of energy and couldn’t go on,” Mayo explained after the stage. “There are times when you really want to leave it all behind. In the end I didn’t quit thanks to my team-mates, who really encouraged me and convinced me to continue. I also thought about the thousands of Basque fans who had come over here. “I don’t know what is wrong, **I don’t have any physical problems, I’m not suffering any consequences of the crashes in the first week.** I’m simply not going well. I am going to continue but while I feel like this I don’t really have any concrete objective.”
http://www.procycling.com/news.aspx?ID=291