Some kids are, as my dad says, 'born with their bladders too close to their eyes.'
They just can't help crying, if the teacher commented in a critical manner, as your son is 6, you can't know for sure, but you know the teacher and say she's nice, I would just make a breezy enquiry at pick time:
Miss X, we've noticed that son is a little more sensitive than usual at home at the mo. Are you noticing similar here? We are working on exprssing things differently, do you have any suggestions?
If she says no, then you can infer tht your son just got the wrong end of what she said. If she says yes, she'll be happy you are addressing it and will likely give you that example and then you could say something like, he's started to get upset that people are noticing/commenting and she will likely approach him differently in future.
It could be growth changes, my youngest, (late teens) got very lively around they age, rough play and silliness, rather than tears, but I can also understand how another child, could have got so frustrated to the point of tears with the little sod, I did when he ran into a stall of stacked apples in our market.