Unwanted transparencies

I've been using Photoshop for years and never had the issue, but for the last few days whenever I've deleted a selection, instead of providing a hard, linear cut - deleting the selected pixels but leaving the adjacent ones as though no change had been made - it now leaves a thin (1-2px?) transparency gradient around the edge. The same issue occurs with Free Transform. I feel like I must've inadvertently selected something, but for the life of me I don't know what.
Thanks for any help you can give.
-Judah

If it helps, upon closer inspection the selection box is no longer even perfectly boxed, the edges are rounded (still set on Rectangular Marquee tool).