It's the nail polish. Any color can stain the nail but red or darker colors make it worse. Applying 1 or 2 clear base coats will help in the future with this like steph4302 said. Good luck.
I guarantee it is from the nail polish, especially if the polish was red/pink and you did not apply 1-2 basecoats of clear polish on first before putting on the color polish. This happened to me when i didn't put on a basecoat and when i took off my red polish. My solution: paint your nails again to cover it up :) it will fade eventually. You most certainly do not have heart disease or at risk of a stroke based solely alone on this. Don't worry. You are fine!
http://i40.tinypic.com/14btjdz.jpg
http://i42.tinypic.com/bipc75.jpg