Thanks to all of you. My back was sore for a couple days, but then felt better.
I was using 3 lb dumbbells. I know that's lightweight, but still pretty challenging for me at my level. I'd taken a foolishly long hiatus from fitness last year and had a nasty back issue in October that convinced me I needed to get my act together and focus on some core strength. Unfortunately at that time, the chiropractor told me to lay off all exercise for a while. (Not sure if that was wise, but I did because the back pain scared me.)
I restarted the end of January alternating cardio and strength (some gentle Pilates --Ellen Barrett/Ana Caban with some traditional strength -10 Minute Solution/Cindy Whitmarsh) and was feeling pretty confident.
I actually had that workout on my schedule for today . . . I wonder, though, if I should concentrate on simple strength moves that maintain neutral alignment since I know my Achilles' heel. Maybe I'll skip that move or just use it as a stretch move with no weights for now.
Thanks for all your wisdom. I need to adjust my understanding of "beginner" to "overweight beginner in mid-40s with back trouble" and work from there.
On the plus side: I was able to do a Pilates rollup the other day that I couldn't even think of doing before. It didn't look as pretty as Ana's or Ellen's, but I did it!
You can bet that was encouraging! I know I'm getting stronger!