Mystery Butterfly at SPI, 3/24/13

I was birding on South Padre Island last Sunday when I came across this butterfly behind the convention center.  Though it never opened its wings, I was pretty sure I knew what species it was.  I had seen a couple of them here last year.  A few days later, after downloading the photos, I thought I would check my ID in the field guide before I filed it away.  Woops.  I couldn't find it.  I checked the Mexican species.  I checked northern species.  After a while I figured out my mistake and my original ID was correct.  So what's going on?