lacking energy

I am experiencing a serious lack of energy today. I ate about 250 calories all day and then went to the gym and during my second mile I felt like I was going to die! I felt really weak, my arms felt heavy and I felt sick. I kept running though. I am just used to being able to run 5 miles without a problem but I guess I shouldn't be surprised. Anyway today I had around 900 calories again and I only burned 400. Fuuuck. Thats too much! I have to do better tomorrow. I really have to.