My 5 year old daughter (our oldest child, we also have a 2 year old son) is normally a very happy child, although can be very sensitive. She is getting better controlling her emotions but one trigger we have not figured out is remembrances of her when she was younger.
Specific stories about her and things she used to do, and videos of her (even from happy memories), tend to get her very upset. Even photos (except those that are hanging up around the house) can trigger a response. She will often cry and need cuddling (usually from a parent) to calm her down. Videos usually have the most extreme reaction (and the video doesn't need to be that old), but stories of what she was like and stuff she used to do can also trigger a reaction.
We first noticed this about 2-3 years ago (prior to her brother's birth). Prior to that she used to enjoy looking at pictures and watching videos of herself we took of her on our phones. When we ask her about it upsets her, she isn't really able to explain why she is upset (and often what she does tell us changes). Asking about why it upsets her when she is calm can also trigger a reaction, so trying to talk to her to get to the bottom of it is not going to help.
My wife and I are at a loss to explain it and we aren't sure how to best handle it. We try to refrain from saying/watching things that will set her off, but I don't think avoidance is always the best way to handle things. Does anyone have any thoughts or input on what might be bothering her when she gets upset?