what can i do?

first of all get or do something to take your mind off everything except what you have to concentrate on, preferably something that you know makes you feel good and happy. then later, see if you can figure out what started it all. sometimes sadness just comes from other places and you absorb it. sometimes it's an old pain that you forgot about and something, a sound, a word, a smell, a place, triggers it to come to the surface and your mind may have forgotten it but your body and heart haven't. relax, it happens all the time. just ride it out and don't panic. it passes.