Weird battery charging display... Charged, not charged. Turns itself off and on...

My 1st gen 5S iPhone will be charged overnight to 100%. I'll take it off the charger and then 2 mins later, it says only 21% charged.

Or it will shut itself off and then restart.

Sometimes, it says there's only a low charge left i.e. 12%. If I turn the phone off, then back on, the charge will read something more correct, which is usually much higher... ~80% or so.

Last year, I took my phone in for a new battery replacement... Was doing really good until about a couple months ago.

Tried to do a search here, but seems like my search skills are lacking. Did not find anything similar to my problems at all.

What the heck is going on?