I had a panic attack today, I got upset at work and had to call my mum.
Basically, something small (or big in this case) can set me off and then I suddenly get flooded with bad thoughts about how all the things I do wrong so I must be worthless (stuff like a spelling test when I was 5, it's really stupid if you actually think about it).
Sometimes I can suppress the thoughts, usually with music, but a lot of the time I need to be talked down - someone has to look after me until I get over the worst and then talk me out the other end.
But it's pretty much exactly how you described it.