I collected a swarm a week or more ago and after a few days I checked briefly and there were eggs, so I was sure I had a mated queen in there. A week after the swarm was housed I checked through the colony. After 3 frames I saw an upside down queen with her head in the comb. Dead with her sting chamber open as the picture shows. My assumption is that she died in a fight but another queen would not usually fly with a mated one. So possibly a queen flew in after a mating flight - although I would expect her to be killed before entering the hive. Or maybe the queen just expanded due to decomposition after she died. In any case, we don't usually see a queen like this!
May your bees read the same books as you do.