What Does A Hard Rigid Line On The Penis Indicate?

i have a a hard rigid line that i can feel along to top side of my penis when it is erect. No pain, no discoloration, and it isn t visible. its a approx 1/4 to 1/2 inch long and feels like an edge, not a bump. it hasn t always been there. I notice it sometime and others it seems like its no longer there. I never used to be there. I m in a long term relationship. STD (if that could be a cause) is very unlikely.