Matt Stafford‘s wife, Kelly Stafford, is apologizing for chucking a pretzel at a fan during the Rams vs. 49ers game Monday … calling it all a "weak moment" and adding she was "embarrassed" by her actions.

TMZ Sports has learned … the L.A. quarterback’s significant other was in the stands during the Rams’ "Monday Night Football" game in Santa Clara, when a fan behind her started trash talking.

According to one witness, Kelly snapped … kicking off an altercation by hurling a soft pretzel at the man.

The witness tells us Levi’s Stadium security eventually stepped in … and escorted Kelly and her crew away from the area to other seats in the venue.

After the game — one in which the Rams lost, 31-10 — when a fan called out Stafford’s wife for not diffusing the situation and letting things go … Kelly issued a mea culpa.

"I’m an idiot," Kelly wrote on her social media page. "He was saying a bunch of things that were aggressive towards our crew but zero excuse."

"I tried to apologize, but in the end knew I was wrong. Will always stand up for my guy and everyone I love, but obviously needed to do it completely different!"

Kelly continued, "I’m embarrassed too! Sorry you had to witness me in my weak moment!"

We reached out to the Santa Clara Police Dept. to see if they were called over it all … but a spokesperson tells us they were not aware of any such incident.

We contacted Levi’s Stadium security … so far, no word back.

We also talked to Matt’s agent … who declined to comment on the incident.

Matt and the Rams are slated to play next after their bye on Nov. 28 against the Packers in Green Bay.