I can't really answer this without casting a chart and this is a public forum. You are a Sun sign Cancer person, so you are moody and changeable, also very sensitive. You could also have other planets in Cancer. Since you only have been feeling this sad for a day or two, it probably has to do with the Moon's movement.
The Moon moves through all 12 zodiac signs in 28 days, being in each sign for around 2 or 2 1/2 days. The good part of this is that the moods can quickly pass as they move in more favorable signs than Cancer. Earth signs probably make you feel grounded. Most Cancers are strong people who bounce back fast, and I think you will too. There are good Moon Sign Books and calendars by Llewellyn which chart the Moon signs for you, and tell you what is best to do on certain days of the month. It could benefit you to learn about that. I am also a Sun Cancer and live pretty much by the Moon cycles. It really helps. I hope you feel better soon. I wrote an article about this, maybe it will help: