Old battery... in need of replacement. Didn't feel like spending more money at the apple store for something I am capable of doing myself (with the correct parts of course).
For the most part this went ok. About four days after the replacement, the battery became unstuck from the adhesive strips (all surfaces were clean before installation and the adhesive strips were not touched by fingers). I re-opened the phone and pressed the battery back down on the adhesive. So far it's holding; but, only time will tell.
The connectors and screws are getting smaller every iteration of the iPhone. Don't be afraid to bust out some reader glasses to get a good view of the workspace and also use a piece of tape (sticky side up) placed behind a cutout piece of paper (to the same size as the tape) to stick your parts and screws onto. You can then label on the paper next to the items stuck to the tape part what and where they came from. Keeps all the tiny parts in place and sorted.