I've had similar when um, I well, um, had not cleaned a lens properly and left a fairly large oily spot that came from what I thought was a clean cloth.
If the negative is real thin as you describe underexposure it still could be an issue.
The first thing that I'd try after checking the lens for cleanliness is simply shooting another sheet. Once does not define a trend. If the problem persists then get serious about trouble shooting.