Is it normal to feel sad all in a sudden?

It's quite normal, well for me anyway, and anything can trigger off this kind of reaction. I can be happy and joking around one moment and the next second I feel like the world has come to an end and I want to crawl into a corner and cry. I have been told that it is down to mood swings but I can't explain it any other way

You can not imagine how many times it happenes to me.... there is always something makes me unhappy and i feel sad suddenly when i was ok.... it is not normal... i think there must be a reason to make us feel like that... i think maybe we are not trying hard enough to find out what it is... !