After Reihenbach, the stress of cleaning up Moriarty's mess, inability to protect Sherlock, and the guilt of forcing his brother into faked death and the whole situation to begin with, cause Mycroft to relapse back into the cycle of comfort-eating and purging that began when he was a teenager.

Eventually, Lestrade notices that something is wrong and finds out what Mycroft has been doing to himself.

Extra hugs for focusing also on the recovery, especially to the fact that there are setbacks as well as progress.

Captcha says: black monday - There will be bad days, but I can promise that it will get better if you keep trying, Mycroft.