Why my become flat? Izzit is abnormal?

Im just 23 year old. Im just find out my really become flat in the half year after i work. This happen after i being work in the advertising company. I work from morning 9.30am until night and sometime till midnight. i spend more time on my work. I got eat a lot sometime at night. And at lunch time, i seldom eat lot. Normally i will eat vegetable at lunch time. I dont get an enough sleep as well. Sometime so sleepy during work. May i know izzit my get flat because i dont get enough sleep? Or can anybody tell me why women can become flat? May i know the reason? And how to get back my ? Izzit my now is abnormal?I wish to get back the shape of my butt. thank u so much for helping me and sharing ur experience.I really will appreciate it.