Sounds like a few things could be happening at once. First colder water is more dissolved oxygen it has. So transversly hot water <80 F has less oxygen. Generally speaking still enough oxygen for fish to live but things happen sometimes.

One key component you mentioned was fact that you have had recent rains. When cold rains mix with hot water that new water becomes denser that and sinks to bottom of a pond. This can cause a turnover which can lead to a fish kill. Have you noticed any reaction of fish themselves. Fish coming up to surface acting listless and swimming on side. Gulping for air at surface, all these can be visual cues that dissolved oxygen levels are low.

As far as not seeing bluegill until a few days later this can be common. One response of bluegill is that they die sink to bottom of a pond and then a few days later gases start to accumulate in fish body cavity which then forces them to surface.

SO big question should you be worried. Yes. Anytime you start seeing dead fish in your pond pay attention to what is going on. Keep an eye on an behavioral changes in your population and aerate your water to help minimize likelihood of additional fish kills.

Fisheries Biologist, Texoma Hatchery.
www.texomahatchery.com