What Causes Nausea And Belching Sensation Despite Having Acid Reflux?

Over the last 2 weeks, upon doing light yoga exercises, I got nausea. Yesterday, I felt nausea again when I had lunch. Tonight, after cleaning the kitchen after dinner, I got nauseated and belched. I also briefly felt slightly light-headed. I have been tired in the mornings when I arise. I have acid reflux and take a prescription for it. I am also taking 2 anti-depressants, one for anxiety and the other for back pain after having back surgery a few years ago. I have had some dramatic kidney stone issues. Two years ago, my bladder was blocked and I had to have emergency surgery in the middle of the night. I am 67 years old and overweight. But I do eat a pretty healthy diet with very, very little red meat and a lot of vegetables, some fruit and chicken breast, turkey breast and some broiled or baked fish. The nausea eventually subsides upon sitting and resting after exertion. Any suggestions?