Struggling to hit / maintain 55mph

First off - I'm really sorry that my first post is a request for help.

I was driving home this evening; and noticed a higher than normal degree of rolling resistance/drag on the car; similar to what you'd expect with a flat or near flat tire. I tried to get the car up to speed (45-55mph); however the throttle resistance became even greater. Further when I got up to 55+ mph; the transmission (or whatever it is we have) would seem to upshift resulting in no ability to accelerate; and a low drone sound. I did notice that even at low speeds there was a good amount of rolling resistance with even very soft acceleration; resulting in the miles/gallon indicator down around 20-22mpg.

A few additional bits of information:
-I did drive the car into the office around 12 hours earlier, with no problems
-There has been a bit of a rattling sound coming from under the car which I suspect was a loose heat shield
-the car has 145K on it
-I checked oil; and it looked OK, last change was +/-4000miles ago
-I didn't notice any drips from under the car
-original batteries
-no warning lights

I'm going to take the car in tomorrow morning to my mechanic; however was hoping to get a few pro tips on what it may be, prior to walking in blind.

Thanks in advance!

 

Thanks @Raytheeagle; pressure in tires looked good - I didn't test them, however based a solid eyeballing, nothing out of the ordinary.

Same old Costco gas;

Not sure about dragging brake pads; however I doubt it - there was a solid rattling when I got up to 50-55 mph; and the tranny up shifted; both which seem to be un-related to brakes.

No recent maintenance aside from the oil change; an no new damage (door dings etc).

 

Thanks @fuzzy1 ; I'll double check tomorrow AM and report back!

 

Thanks @FuelMiser

Actually just as a quick update; no word back from my mechanic. Fortunately we have an extra car; so I'm not in a rush to get the prius back.

The cat coming loose sounds like a very real possibility; as I've had a bunch of rattling coming from the bottom of the car. I thought it was a heat shield however it very well could be the cat.

What does "ICE" stand for?

 

PS: Thank you all for the suggestions. Very much appreciated. I'll update once I have a resolution!

 

Afternoon all -

Got my car back Mechanic ended up checking the cats which were ok; however the 12v was the problem.

I ended up doing ignition coils, tranny fluid; and spark plugs since I was there.

Car runs like a camp.

Thanks @FuelMiser @Greenteapri @Robert Holt @Mendel Leisk @scona @Raytheeagle for your insight on this one Happy holidays!

 

So seems like I was too quick to claim victory on this one. I think I still need to do the cat. Which is where the rattling is coming from