Weird

Lately about once every week so far this summer, I have been waking up with a blue marker line, somewhere on my arm. The weird thing is, I have not used a blue pen/marker,  or intentionally marked myself. I dont know how this is happening, Ive thought of everything. My laptop case is blue maybe the color is wearing off? My walls are blue, maybe ive smacked my arm on my wall and somehow the paint came off? Everything Ive thought of doesn't seem as a good enough reason for the marker line. First of all, laptop case? not so much, i tried to see if the color was coming off...nope; and my walls? please the only time paint will come off, is if the walls are wet. The marking is always a Perfect line in a random spot on my arm, this morning on my wrist and forearm. My last idea for this weird phenomenon... Ghost? honestly I cant think of anything else. 

Do you have a logical reason for this occurrence? Let me know...comment!