Miranda Lambert -- a finalist on the 2003 season of "Nashville Star" -- is accused of assaulting a woman just because she didn't know who Lambert was!

Aisha Esbay, the alleged victim, tells TMZ she was sitting at a bar with friends when one of them recognized Lambert and wanted a pic. Esbay approached Lambert, said she didn't know her but her friend wanted a photo.

Lambert, Esbay says, was pissed, and shoved her into a bar stool -- multiple times.

Smith County Sherriff's Dept. tells TMZ they're investigating. At worst, they say, it will be a Class C misdemeanor -- kinda like a violent speeding ticket.

Lambert's reps tell TMZ they're in the dark about the whole thing.