While I realize this may be difficult without a photo, I'm hoping someone can say "This is a list of possible species given your location and description" and I can attempt to find a photo that matches from there.
I'm in NE Indiana. I think it was an insect. The species in question looked a bit like a leech, black and shiny but wasn't segmented and it was a bit flattened out (but not entirely), maybe an inch long. It wasn't tapered. Also I don't believe it made any attempt to bite me (or just wasn't around long enough to). Unfortunately it startled me so I smashed it pretty quickly so I didn't get a picture, but I got a good look and couldn't identify any legs, eyes, antennae. It moved fairly quickly, almost in a pulsating motion. When I smashed it, there was only a bit of white, mealy paste left behind, the body almost seemed to liquefy.
If it hadn't moved so quickly and wasn't so small I'd have thought it was a baby slug. It's possible it could have been some sort of larvae.
I also don't think it was a leech because I found it on my body but I was in my home and can't think where I could have been that would have exposed me to one. I noticed it when after removing my shirt and lying in bed. So it was either inside my shirt for several hours without me noticing, fell out from the covers when I got in bed, or fell from somewhere after I got in bed.