My mac change the hostname when this message appears the hostname is changed to the machine, the text of the message says

Your computer name "C - " is already used on this network. The name was replaced by "C - + 1"
This is an issue with the network and the other devices. 

The bonjour sleep proxy function allows other computers and devices to listen for your Mac while it sleeps and answer. When that gets confused, the other device “claims” your Mac name and the only thing your Mac can do is make a new name. 


Computer name keeps changing OSX 10.10 (14A389)


You can confirm this shutting off every other device in the network and changing your Mac and then sleeping and waking it a few times. 

Long term, figuring this out isn’t super easy, so knowing it doesn’t hurt things in general, can you live with this knowing why it happens?