New battery, car idling then lost power

Hello,

On Sunday just gone, I had a new battery fitted. The entire experience went well.

However, since I have driven my car for two short journeys each on Tuesday and Wednesday and just now to the supermarket about 15 minutes away, the car has been driving well so far but when leaving the supermarket, the revs were not just staying in one place, they were kind of going up and then down a little bit.

I managed to get through the majority of my journey with the car functioning as normal but this rumbling-like sound with the revs resumed whenever I had to stop. At a traffic light, this caused the engine to cut off.
I momentarily put my hazard lights on and then luckily by safely turning the car on and off twice I believe, I was able to drive the short distance back to my location.
When this happened, I had two warning lights momentarily, the battery and I believe an engine warning light.

When I had safely parked at my location, I turned the car off and then on again and this rev rumbling had stopped, and I had no warning lights?

Just wanted to query if this could be related to the new battery? Or if you had any advice for when I drive next or what this means please?

Thank you