Sudden decrease in MPG, car sounds different

I have a 2006 that I bought used about 6 months ago. Had some issues when I first bought it with the traction battery, but those eventually resolved themselves and I figured they were due to the car sitting in the dealer's lot for a month. Since then, I've had no issues with the car and have been regularly getting 50-53 mpg.

About two weeks ago, I had a weird incident where I went to drive home after work, and the car was noticeably sluggish, like the engine was working extra hard but the hybrid battery wasn't doing anything. After driving for about 30 min it went back to normal. It was around 95 F outside that day so I thought maybe the battery was overworked/overheated.

The same thing happened again this past Friday, even though the weather was closer to 85 F, so I suddenly wasn't so sure it was because of the heat. I had, however, driven a short trip to lunch that day, sat in the car with the A/C running during lunch, then drove a short way back. I figured maybe it was a combo of the heat and the battery being drained from the lunch trip.

Then yesterday, I took the car on a long trip, about 200 miles (probably not the smartest thing to do if I thought there was something wrong with the car!). There were more issues like the day before, where the battery clearly wasn't pulling its weight. I was blasting the A/C at max, however, and had a full car of 5 people. So I convinced myself it must be THAT.

Finally, today I was driving around a bit, trying to see if there's something wrong with the car or I'm just being paranoid now. It seems like my mpg have dropped to around 45-48, which is concerning since I was getting ~52 literally two days ago. And this may just be me fishing for problems, but the car seems to sound different. The engine doesn't seem as loud as it was before when I accelerate, and I can't really tell if/when the car is in ECO mode vs the ICE being on. It used to be subtle but pretty noticeable if I were paying attention. Now I can only tell if the engine is off based on the mpg reading on the MFD.

Again, maybe I'm just being paranoid here, but these couple of incidents have spooked me a little. Nothing else seems to be wrong with the car, and I haven't had any warning lights. Maybe it's the 12V battery?

Sorry for the long story, but any ideas what this could be? Any advice is appreciated. Thanks.