Although I wouldn't bet the farm on it (or my life), I would guess you might have something in the Panaeolus genus, perhaps Panaeolus subbalteatus- commonly found on compost or manure mulch in gardens in warm, humid weather. As Devon said, more photos of the gills and of a split mushroom will aid in identification. The spore prints of this group are jet black. To obtain a spore print, remove the cap from a mature mushroom, lay it on white paper, and cover with a glass jar. In 24 hours, you should have a spore print color to help with identification. Good luck (and when in doubt, throw it out).
Dorie, thanks for the 'spore print' thing. I am out of town at the moment but I did just have some shrooms pop up from where I moved the turkey pens. I'm not planning to eat them but it would be cool to find out what kind they are.