I know how you are feeling. There are many people who feel just the same as you do. You can have depression, hormonal changes, not enough sleep, etc or a combination of these that will effect you this way.
If you think about what is going on in your life, there are probably no really big things that seem worth crying over, so this makes you feel like your urge to cry is unfounded. But with whatever your condition is, even little things are going to make you upset: tiny little things that are so trivial, you don't even notice them, and it may be just one, or it may be so many more than one all lumped together, that make you just want to cry!
You are most likely an introverted person, so you let things build up inside you. Sometimes all it takes is something that happened about a month or two ago plus one more little thing now that can set your feelings to want to cry.
If you need to "let it out" and can't find a reason, just remember that it is there, and there is nothing wrong with doing some crying to relieve the tension.