What is the reason behind missing period

I want to know i missed my periods last month it happened with me first time i work in night shift in Bpo my daily routine got lots of changes and food as well. When i was in my hometown all thing was ok when i came to Delhi this happened with me first time and my face skin is so rough seems acne.I have lot of stress as well and my urine colour is yellow and going to lot of motion in some intervals.I eat too late as well like in morning 6am and late night at 3:30 or 4am whenever i get time my feets are swollen as well as i got sitting job and i work at least 12hours per day. When i constantant to a doctor she given me same tablets but nothing comes up as results as i told her she said i have go altrasound which seem I Am nt able to understand for what she needs ultrasound what she is trying to find out when i am telling and sharing each points.So i need a doctor who can help me and considering i am not pregnant. I hope some one can help me. And most important i am put on weight as wel