iPhone 6 battery has been acting wierd

This might be a stupid guess, but I wonder if the iPad charger is pushing in more power than the original iPhone charger. Happens a lot with my other stuff for instance, I have a charger for my bluetooth headset which is giving the power a lot slower, which is kind of typical because it's a quite small headset, and my brother and my mother used that charger to charge their phone and tablet, they had to leave them to the charger for almost an entire night to fully charge.

I had that happen with my iPhone 5S when the battery was starting to fail. I vaguely remember the battery sitting at like 20%, then dropping to 1% and eventually dying. I then had to plug the phone in and wait for a half hour before the thing would come back up. Best guess is the battery gauge is not calibrated. Run the phone down till 5% or so, then plug the phone into your charger, and let it charge. iPad chargers do supply 2.1 amps of current, but that isn't really the issue. You can charge phones using that faster charge current and it won't damage your product. Chargers like that one will determine what the device connected to it needs and choose accordingly.
HTH

Actually, run the phone until dead then leave it alone until it cools down to room temp. then plug it in and don't use it until the phone is fully charged. I do this once a month, and my battery has always ben in very good condition when it comes time to sell it.