blood under the nails

we can bleed

I had the experience, not under the nail (without trauma), but in the hand, (and subconjunctival in the eye, a couple of times) so yes it could be like a little stroke, or bruise!

a bruise is caused by broken blood vessels leaking blood into the tissues. It, the spilt blood, then tends to work its way through the body, therefore to the surface where we see it - thus there is usually a delay before we see the bruise.

Under thenfingernail though, the bruise doesn't get to dissipate as easily.