Yellowing Ink?

These two sketches I created right before an open studios at Long Beach. Unfortunately, I left them out for people to look at (even though they didn't). Short story long, they changed from black Bic ink to a yellow shadow. I tried to re-draw them but it seemed as though the ink I put on it also changed to yellow. I don't know what the hell happened, but I've never had this problem before. Oh, and to make my story less credible, the yellow some how didn't even show up in the scan.