I'm sorry you're going through all that. It does sound like depression/stress. I think I experienced one somewhat serious episode of depression in my life about seven years ago, and there was certainly a lot of crying for no reason and nausea involved. Also tends to be the case when I fall into more minor depressions - I think I have a very slight tendency in that direction.
Best if you can speak with people you trust about how badly you're feeling, and/or see a doctor. I hope it gets better soon. In my experience and those of my friends who've had similar times in their life, it does eventually get better, partly through the passage of time and partly through taking whatever steps you must to get better. It's just when you're in the middle of it that it feels like bloody forever, which is very very hard.
Also, what Vasilisa said about being able to help others somewhere down the line, with the experience you've had... It is so true. It's one of the few reasons why I (in a weird way) am glad I have some experience of really dark times in my life. Being able to empathize that much more with others is a gift, when you get there eventually.