Thin brown line on finger nail

Hi, I noticed a pale brown thin line appeared on my 

finger nail,  The colour got stronger and I went to my local gp referred me to a dermatologist. While waiting on my appointment I noticed another pale brown line appearing on my thumb. I seen the dermatologist today who told me to take photographs and come babk in three months. Anyone else had this experience? I have had no trauma to my nails and do not wear gel nails.