Are there any reasons for splinter hemorrhages other than the heart? i have tiny ones in the middle thumbnail. no trauma, no pain with them. they just show up. thank you?

The main cause of splinter hemorrhages is trauma (often minor and not noticed) to the nail. That it is happening with only one finger is strong evidence against the heart of any other internal organ or illness being the cause.

Commonest cause most likely is minor injury. If no likelihood of injury, would get an examination including basis lab studies to be sure.