Since you noted it was an older woman, and my experience has been that is one demographic that, if anything, seem to be overly cautious around cyclists... I'm gonna go with she thought you were going in the same direction, merging onto the spur road.
Or else, she never saw you at all.
Regardless, as others have said, that type of intersection is one I would be very, very careful assuming any car knows you plan to go straight across. I have ONE route I ride that I cross an on-ramp entrance to a 4 lane highway. I always watch what is coming up behind me. Thankfully, it is a very light traffic area.