Why did I gag???

I have the most sensitive stomach lately. Everything makes me gag. Brushing my teeth, after I eat certain foods. Yesterday I swatted a fly and almost threw up. I have never done that before and have swatted hundreds of fly's. What could cause me to be so sensitive all of a sudden? I'm on Metformin and took a test a week or 2 ago because I was bleeding a little and it was negaitve.