Why am I always lightheaded and nauseous?

For the past year I have been having problems with being lightheaded and nauseous. It does not happen everyday but it happens enough that I am miserable. It comes on suddenly and sometimes last only half the day but there are times that it lasts all day for a few days straight. This morning however, I had the same symptoms but I also felt very hot. This is another new thing. I am constantly sweating so much that shirt gets soaked with sweat. I have been on Bupropian for a year. I have three children and I am stressed everyday but I do not have these symptoms everyday. I eat pretty regularly and healthy. I do smoke but I have cut down to 2 cigs a day. I am working towards quitting. I know that I am not pregnant because I had Esure done a year ago.(springs in fallopian tubes, scar tissue grows around the closing the tube completely) I did go back to the doctor and got tested to see if my tubes were 100% closed and they were. I am a stay at home mom. I do not know what this could be. Diabetes run in my family and I have an appointment in June for my check up and my yearly diabetes test. What can this be?