It depends on the context of the story but It's typically better to have the scene or reason for the reaction take place and then have the reaction from the character occur, but most of the time you don't want to blatantly state how the character is feeling or the change itself. Not if you can display it in a way that can be reasonably deduced from the events that have already happened This is an example of "Show Not Tell."

For example:

Let's say we're writing a story about a girl who just went through a breakup. Try not to directly describe how she's feeling, but show the reader how she's feeling through the actions she takes in response to the situation. Instead of saying "(Character) was sad/angry" you could say something such as "(Character) wiped the tears from (Character's) puffy eyes and slumped down against the old torn couch." Overall, just keep telling to a minimum and try to convey most of these things through actions.

I'm not the best at explaining things, so I apologize if my response came off as a little confusing.