I have two vertical black lines on my left thumb nail also one on my right thumb nail also i have a pain on my left side os stomach i've been under?

Melanonychia: If melanonychia, or dark streaks on the nails, is secondary to systemic and/or dermatologic disease, treatment of the underlying condition is helpful. If melanonychia is secondary to a drug, discontinuation of the offending agent may result in clearance. In rare cases melanonychia can be caused by melanoma, a form of skin cancer.

Get checked.: 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 checked out when in doubt.