Gee, so young, that's sad. The symptoms you mentioned sounded very much like some I've had that had a bacterial infection, but they were "off" for several weeks, they gradually layed fewer eggs and the shells got thinner over those weeks. One produced a lash egg and that was the definitive symptom. Comparing notes with other peeps it's obvious that birds from commercial hatchery stock comprise a much higher percentage of layers with similar symptoms, including being egg bound or having a prolapsed vent.
Did you do, or consider having a necropsy done?
Maybe there are just more hatchery birds out there versus homebred pure so to speak. One of my chickens died last year, she was a SLW from a feed store, she was one year old, showed no symptoms, laid an egg that morning, no change in egg quality, died in the afternoon. It was definitely hot that day but it had been much hotter earlier in the summer. So I have no idea what happened to her. All I know is it is very sad when you lose one.