X17 XCLUSIVE - Is John Cheating On Jen?!?

Jennifer Aniston isn't going to like this!

John Mayer spent the night at her place on Monday, but on Wednesday, her loverboy took an unidentified blonde woman to dinner at Marix Tex Mex restaurant in Santa Monica, and he appeared to be very friendly with her!

After stopping by Ren-Mar studios just a few hours earlier, John met the beautiful blonde at the eatery around 8pm, and an X17 photographer on the scene told us, "John seemed a bit nervous, which is unusual since he's usually so cocky around us. The two of them left separately and they basically ran out the door to their separate cars."

The photographer added, "John was talking to the girl, who was really cute - she was very animated and excited. It seemed like he was telling a really great story, and as the dinner went on, John moved around the side of the table to be closer to her. If I were Jen, I would be pissed!"

Check out the video below to see for yourself!