Why does my stomach hurt?

I went back to starving myself today, I probably had around 250 calories all together. I know that some people are going to discouarge me from doing this, but I walked down to a library and I drank a diet pepsi, and my stomach started hurting a lot, I felt queasy and when I got home I threw up a little (not on purpose). But I'm worried and I hate when my stomach hurts like this, I have starved myself before and it never felt like this.