These sound like splinter haemorrhages. These are small bleeds under the nail caused by trauma, as the nailbed is not smooth but has tiny little grooves and rails in it, the blood lies in the grooves, and looks like a splinter....hence the name.
We usually see this in people who are hard on their nails, wear them too long or pick off enhancments causing trauma, or of course nail biters.
It is nothing to worry about, these marks will grow out, but to stop anymore you need to be a bit more gentle with your nails, and of course cuticle oil will always help :wink2: