I've noticed that Thais are quite moody people.
The seller at the market who always tries to strike up a conversation suddenly doesn't even look at you.
I wouldn't read too much into it, his/her mood probably had nothing at all to do with you.
Maybe if you would have smiled, you'd have gotten a smile back.
I've even had some bad vibes with a neighbor and the next time he smiled at me!
I did give him a nod. As it goes, I got exactly the same non-reaction with another neighbour a few days later. Anyway, this thread taught me this is an Asian thing, not specifically Thai. I do agree Thais can be moody though, but that's probably another topic entirely and one that is, quite frankly, rather boring for all its predictable negativity and imbalance.