Your story is not unusual - shift work is easier when we're younger and our bodies are more flexible in all ways. You need to talk to your employer about possible straight shifts for a while to see if things improve. When you get off nights, try to limit your exposure to bright light-wear sunglasses when you leave work. When you're on days, the opposite - open the blinds and get as much bright light in the morning as you can. Keep yourself on the same sleep cycle as much as possible.
Shift work gets to everyone after a while, some more than others.
I am not overweight by any means, in fact I am underweight. About once a month I wake up gasping for air. I cannot breathe and it takes about 30 seconds to be able to breathe again. My husband usually gets up and gets me a glass of water and this helps. The official answer is that, yes, is could be a form of slep apnea.