Hi I Have Got A Grey Line Running From Right

Hi i have got a grey line running from right tumbh, the cuticle down the free edge. I have had it for around 4 weeks, it apperead the day after i had a knee surgery. It is 1 mm in witdh, has not changed over the course of the 4 weeks. I have noted other lines on my left hand fingernails, however they are much more fainter and not as strong colored. What could the line on my right hand tumb be?