Mikaela Spielberg got busted Saturday night in Nashville, after returning home with her BF from a bar. Cops were called after an apparent argument erupted between them after the dude said something "rude" to her, which allegedly resulted in Mikaela hurling objects at him ... one of which struck him in the hand and left damage.

The responding officers saw a visible injury on the BF which looked fresh -- they say there was dry blood on his hand and that his wrist looked swollen. The police say both parties admitted to a physical dispute but determined Mikaela was the aggressor.

One of the reasons they ended up booking her on domestic assault charges is because cops say her story on what exactly happened constantly changed, while cops say the BF's story remained consistent. They make a point to say the guy didn't want Mikaela prosecuted, but the officers say their department is still moving forward.

Mikaela was booked and held on $1,000 bond, and has since been released.

Of course, the arrest comes shortly after news broke that Mikaela's forging a solo career in porn.