The woman accusing Nick Diaz of domestic violence claims the UFC star has been using cocaine ... and that's what sent him into a violent rage ... this according to law enforcement sources.
Multiple sources connected to the case tell TMZ Sports ... Diaz's accuser claims she's been in an "on-again, off-again sexual relationship" with Nick for years. But it took a turn for the worse in the last few days ... because she claims Nick has been using cocaine.
The woman told police Nick attacked her at a Las Vegas home Thursday night .