2 things to say:
1.  We only do good thing - treat ppl nice - when we want to tackle them.  So that means we have expectation and put some unwanted stress into the equation.
2.  When we have expectation and when things do not go our way, we feel sad and sulk about it.

So that is why you are so moody when you saw that remark.

Lastly, though it may not be a wise move, if the remark is eating too much into you, just braven up yourself and ask her.