do a compression check, remove spark plug and check compression, write down compression number, spray some oil in the cyclinder and do compression check again. If numbers are ok or the same than move on to the Fuel/air system since something got screwed up in there.
If you did any mods than go back to stock first to see if there is any change.
Since the season is changing there might be a change in the air density which could cause performance change.
That is why when I get my Grom I'm going to buy me a note book and document all the readings on the Grom before I ride it such as compression check, voltage output, shock sage, etc and I will ride it for a month and check and document the readings again just to see if there is any change.
When I start to mod the Grom such as counter sprocket going to 14T I will check the speed and gas mileage to see the changes, same thing when I replace the stock tires to Michelin.
My document information will be used to see if I see any changes in performance, mpg, handling, etc.
Ride safe.