Unfortunately it's common. I've seen several people do this and my own sister is one of them. She's almost 40, about 20 years out from a hospitalization for anorexia, and eats a very strict vegetarian (sometimes vegan) diet, suffers from ambiguous "stomach problems," has become a runner, goes to great lengths to get sometimes 2 runs into her daily schedule, and has built a life around being able to over exercise and control control control every aspect of her routine. She has many people fooled. Her husband finishes her food for her and she eats so very little it is hard to understand how she can muster the energy to run, but when she is not exercising she is very still and often complains of being uncomfortable and wants to sleep. When odd (under)eating behavior is coupled with horrible anxiety, which can manifest as a bad attitude, it's likely just another form of the eating disorder taking over. It is so so so tremendously sad.