Hi BellaChica,
A 20 day cycle is not impossible. It's quite possible to have a period twice in one month. Some women have a period every 20 days for months on end. What is your normal cycle time?
Another possibility is that you got pregnant but had a spontaneous abortion--that is a natural one. These are extremely common (and happen because something is not right with the embryo or something). I think something like 60% of human embryos don't "take." That would also result in an early period. Most women just think they are normal periods that are a few days early or late.
You had sex without birth control on the days you were most likely to ovulate. Are you trying to get pregnant?
Unless you explicitly want to have a baby, you need to start using birth control. Do you have a doctor you can go to? If not, try Planned Parenthood. Also, how old are you?