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Amber Portwood is vehemently denying her boyfriend's claim that she went after him and their 1-year-old son with a machete ... she says his version of events makes him sound "insane."

Amber finally opens up about her domestic violence arrest on the 'Teen Mom OG' reunion show, telling Dr. Drew her side of the story and ripping her boyfriend, Andrew Glennon.

In this clip from the special, which airs tonight on MTV, Amber says Andrew's clearly off his rocker for trying to make her out as some sort of machete-wielding maniac.

TMZ broke the story ... Amber was busted for domestic violence July 5 after Andrew contacted cops and claimed she struck him with a shoe while he was holding their child, James.

We later learned Amber also allegedly wielded a machete and struck a door with Andrew on the other side.