I've noticed small/thin black lines keep appearing under the tips of my fingernails over the past few days. what could this be from?

Splinters: Thin longitudinal black lines under fingernails can be what doctors call "splinters" & can reflect tiny blood clot emboli that get caught there, where there are end arteries & capillaries. Such tiny clots end up there after being generated inside left chambers of heart. Cardiologist exam indicated & urgently if dizzy, feverish, chest pain, irregular heart beats or shortness of breath.

Splinter hemorrhages: That is just a guess. Black stripes down one nail could be a subungual hematoma. Red or brown streaks could be splinter hemorrhages. Brown/black lines from the nail plate to tip are often seen in people with dark skin & are normal. Subungual Melanoma can cause nail ridges and dark colors of the cuticle. It usually begin in the nail matrix. It most often effects the thumbs or big toes. Get check