Rectangular marquee tool

Hi. I've hit an odd problem. Every time I try to use my rectangular marquee tool to select a portion of an image so I can crop it, I get the Warning: No pixels are more than 50% message. This isn't happening if I try to use the elliptical marquee tool or the single row or single column tools. I've checked the opacity of the image and it's 100%. However, just to see what would happen, and in case it had something to do with the background layer, I tried creating a new layer and filling it and it only filled at, I'd guess, about 50% although all the settings were telling me it was at 100%. The other thing I've noticed is that, although the image is 640x360, if I try to scale it (just to play around and see what would happen), it's showing me a much bigger canvas size. It seems to me that a couple of things are happening. One is that somewhere there's an Opacity option accidentally left checked and, possibly, for some reason, Photoshop things the canvas size is a lot bigger than it actually is. And, again, this problem is only happening with the rectangular marquee tool. 

In the meantime, I'm using the crop tool but I'd really like to sort this out. I have done a search and the most recent answer deals with a different question.

Thanks a bunch.