So just to update...
I bombed the workout. It was 4 sets of 5x200 at mile pace. My 1600 PR is 4:52 (which was gonna go wayyy down this season!!) and I hit 39-40 for the first set, feeling awful of course. I talked to my coach and stopped there.
I didn't try lifting; my coach says to do easy runs for a few days and try lifting on Friday. He seems pretty relaxed and keeps saying I'll be fine. He says that, even though I feel much weaker and my arms are significantly smaller, that the strength will likely come back pretty quickly, within two weeks. He compared it to not lifting for two weeks, though I think this is a bit different because my muscles are depleted now.
I'm not panicking like I was yesterday but I am worried. Mostly though, I'm confused/disappointed right now. I relied on my body to give me feedback about whether I was eating enough and I feel it misled me. Although I was hungry more often, my workouts were going well and I was losing fat. I had a very solid workout as recently as Friday. Then suddenly, my arms shrivel up, I feel weak, I fall back on an easy run, and I have lost even more weight.
Since it happened so quickly, I'm really hoping I can return to full strength - literally - soon. I will post at least one more update for the benefit of people who use the search function in the future.