It sounds like you have bracketing on anyway. One press of the shutter button is one exposure in the bracketing sequence. The next shutter button press will have a different exposure. The top LCD display will show a BKT indication if it is on.
It has got to be worth checking.
I am very guilty of starting to shoot without checking my settings and not infrequently find that I am using a silly ISO or have bracketing still on or an exposure compensation applied. Gradually I am disciplining myself to reset all the main settings to my "usual" (Aperture Priority, ISO 100, no bracketing, no exposure compensation) at the same time as I format a card after downloading from each day's shooting. But I still sometimes forget and then get a surprise next morning!