Loss of lower leaves

I set up my first planted aquarium about three months ago, and until now things have been relatively problem-free-- if anything, the plants are growing *too* well! However, I've just returned from a week-long vacation, and was surprised to find that my Giant Hygro had lost most of its lower leaves while I was gone. It's been growing really well up until now, and I'm not sure what changed in that one week. A few guesses:

--I pruned the Hygro for the first time shortly before I left, as it was getting overgrown. All I did was just lop the tops off the two main plants and replant those nearby, but maybe that had something to do with it?
--I haven't added any root tabs to the substrate around the Hygro, since it seemed to be growing great without them (unlike my swords, which are doing sooo much better now with the added ferts). Could the Hygro have simply exhausted all the pre-existing nutrients in the substrate at this point?
--In anticipation of getting a pair of Blue Rams, I increased the tank temperature from 72 to 78 degrees about two weeks ago. I tried to do it very slowly, increasing only a degree or two per day, but even that may have been too much too fast.

Does anyone have any ideas? I'm really new to plants, and want to try to learn from my mistakes!

Thanks, y'all. I hadn't thought about the light issue, but that makes a lot of sense. I have really low lighting anyway (just over 1 watt/gallon-- can't afford to change it at the moment, alas), so the light penetration to the bottom leaves is about nil. I did as suggested and replanted the cut-off tops in front of the bald spots, and it looks much better.