What does it mean when the battery icon turns yellow after you charge your phone?

It is an indication that you have turned on the Low Power Mode option.

I may have or is it possible that my device can go into that mode automatically as indication of anything specific? over use and/or over charging in short period of time?

do you recommend changing it back?

Will it affect anything negatively or can I keep it as is and will it switch back auto? Curious

Thanks!

kbf-iCloud

It will have no negative impact on the battery.

Use Low Power Mode to save battery life on your iPhone - Apple Support

You’re quite welcome.