Sounds pretty harmless Mort - I've re-read your post a couple of times to try to figure it out. It could be a case of coming out of that "auto-pilot" effect abruptly mid-journey.
Keep an eye on it & if it happens again & worries you then mention it to a doc.
I often break off from working to persue some random thought, then when I look back at the computer screen I can't for the life of me remember what I was doing before I stopped.
I also sometimes forget what I'm talking about half-way through a long rambling sentence - I think I just bore myself into unconciousness on those occasions......
Don't dwell on it - just bear it in mind if you get any other symptoms.
Stress (ongoing stress, or the transition between stress/no-stress & vice versa maybe) does sound like a likey suspect....
Hope you're not letting it worry you.
Kris