Did you meter for ISO of 100? ISO, shutter speed and F-stop are the three variables of a metering system. You input two and the meter gives you the third.

Are you shooting digital or film. If film, are you having it processed at a corner drug store (read that as one hour photo lab) and are they automatically adjusting. There might be nothing wrong with the negatives except that they haven't been printed properly. If you're shooting digitally, you can easily check exposure by looking at the LCD on the back of the camera to see if it's in the ballpark. If it looks good on the LCD, does it match your calibrated monitor. If your monitor isn't calibrated than the files could be fine but your monitor is off.

There are a lot of reasons you could be having a problem and you would need to post your work flow to see where the problem might be.