No matter what setting I have my camera on, any time I use the flash the exposure goes super high. I try changing my ISO and aperture, too, but it doesn't help. I have a Nikon D3400, could this be a sensor issue? It was working just a few days ago and I picked it up again to find it completely messed up. As long as I don't use the flash it works perfectly, though. What could be wrong?
When you use the flash in a non-Auto mode (Aperture or Speed priority) the camera assumes the flash is used for fill-in and computes the exposure for a flash-less photo. So if you subject is dark this will crank up the exposure and then you'll have a big flash added. So when you use the flash, either go full-manual, or use the "Program" mode of your camera.
I have a d3300 and sometimes I had the same problem. If you take the photo in almost total darkness the camera can't calculate well which flash output to apply. Also, these cameras have a max shutter speed of 1/200 with flash and it ends in burnt or overexposed photos easily.
Use manual mode for flash, reduce its power and also use smaller apertures.