Stroke randomly/intermittently changes to transparent when creating line segments

I don't know if this is a bug or user error. I have not been able to find anyone else having this problem. I draft sewing patterns in Illustrator and use a lot of line segments. Many times when I am creating a new line segment by entering the measurement, the stroke and fill will randomly change - I always set the fill to transparent (or no fill?) and the stroke to black. What happens is that the stroke often (but not always) changes to transparent, and every now and then the fill will change to black but only rarely. When this happens I have an invisible line segment that I have to change to black stroke. What am I doing wrong?