Anxious!

[cothes]

Hello, I am a 6'5 ft tall guy. I weighed about 115-120 Kg for the past 21 years of my life and now weigh about 108-110. At about this year I started missing weight mysteriously.I am a medical sstudent who started combining work and studies and sometimes get to eat once a day. I also get a lot of bowel movements in my stomach and sometimes like chills running through my spine with noise. At first it was intentional as I went on a military diet for a week and cut out White bread, Soda and pizza from my diet and I could walk for about 7km almost everyday. What makes me worried is that I stupidly had an affair and tested nehative to Hiv but i am scared to death may be the doctor's made a mistake because it is 6 months since this started and I had this experience a year ago. My questions:
I have Acne also could it be connected?
Could it be connected to stress?
Could it be Hiv!
Could it be Git disorder.