If you have a capo, you could test it on the first fret (or second, or third, etc.) and see if your problem stops. I had a guitar with a worn nut that put a couple strings just low enough to always buzz against the first fret.

Other places to check for buzzing include:

• The ends of the bridge knots. These should have been tied so that they are looped into the knots of the other strings, but if they are sticking out and touching the soundboard you may get some vibration.
• Same of the ends of the strings around the tuning pegs. It looks like these have been trimmed so that there isn’t any extra string after the knot around the tuning peg, but if there is it will sometimes touch the headstock and buzz (I don’t trim mine, so this happens to me sometimes).
• Someone else mentioned that your strings are touching the wood due to how they are wrapped - also a possibility. I’d loosen the strings and push those wraps further onto the post.
• And just a wild guess - were you playing while wearing a shirt with buttons?