What is this black splinter looking mark under my fingernail?

I have a black splinter looking mark under my right index fingernail.. its not at the end so i know its not a splinter. I had one on a different finger a month ago. Any ideas on what this could be? Iv been reading about it and i read that it can be caused by endocarditis.. thats really scary! wouldnt i have more of them if that was the case?? i recently had some bloodwork done for panic attack symptoms and it came back normal and my EKG was normal so wouldnt they have found something then if it was serious? or maybe not because they didnt test for that certain thing... anyone with any word on this please help me out...