Eating disorder? Brother (M27) spits food up in a cup, what could this mean?

He has low BMI. I don't even think he realized he did it but found a cup whiled with food in it and I'm just puzzled. He's never said anything about having an eating disorder. He has anxiety. He skips meals sometimes. Gf tells him she hates the food he eats. Like he loves pesto and eggs. When I see him, he does eat. He goes to the gym. I just don't understand why me and our mom found a cup of chewed up food. It's weird. Could he be hiding a disorder? Usually he tells us everything, even embarrasing stuff, when he's stressed.