Great advice you've given, Carol.
I agree that it sounds like you're slipping a stitch. It looks like one stitch that stretches up to cover two rows? It's not especially loose, right?
Just stretched up so it looks tall? That's a slipped stitch.
It might be that on your DPN's, a stitch has fallen off the end of a needle (common), and when you put it back on, it's run down a row without your noticing, so you're putting on the stitch from the row before. This would probably give you a bit of a loose loop in the back of the work, where there had been a stitch, and then it was let out.
If there's nothing loose back there, and everything is pulled taught, then you're probably slipping it as you work the knitting: meaning, you paused in the middle of a stitch, because you were distracted by something, and then, instead of continuing by pulling the working yarn through the stitch, you think you've already done that, and you slip the stitch off the left needle, and continue from there. My guess is that this is what's happening; so just be more aware of where you are, when you get distracted by something.