Is my lack of food causing this muscle pain?

i havent been eating properly for about 9 days usually having under 500 calories a day. i have lost 7 lbs . im 16 stone 3 (now) and 5"11 tall and 18 y/o and female. today i have been having muscle pains down the front of my thighs and across my chest. it feels like i have pulled a muscle but i havent done any excercise or activity. what i want to know is is it my body eating muscle for energy ? but why doesnt it use the fat? i have about 6 stone spare so it should be using that for energy right?