Sudden change

I'm not sure how it came about. One minute I was feeling happy, calm and normal, then the next... there was this kind of feeling, like I was sad about something. It was while I was hanging up clothes, and my mum was talking on the phone to her sister (my aunt)... I don't really know how to describe it, but it was like a squeezing feeling, below the rib-cage...

I wish I knew why this happens, it's been happening a lot lately...