C182 pilot reported they received a terrain warning while entering the pattern at a non towered airport and after landing discovered their altimeter was off by 450 ft.

Narrative

I was moving my airplane from an annual at ZZZ1 a very shot hop from my base at ZZZ. I set my altimeter before takeoff and ensured it matched field altitude. On my way to ZZZ; I did a few steep turns for practice; so took a little longer than anticipated to get back to ZZZ. Everything was going well. Traffic was busy at ZZZ and the procedures have recently been updated to accommodate for the increased traffic. It's challenging due to being tucked under the Class Bravo of ZZZ2. I crossed midfield at 1500 feet (500 above the pattern) then proceeded out under a 1700 foot class B shelf. My main concern was not entering the bravo- it is under final approach for ZZZ2. This is when I first noticed that it felt low; but I hadn't flown this new procedure yet; so didn't have a sight picture reference. I got a warning from my navigation system that I was 500 feet agl and increased altitude as much as I felt comfortable under the Bravo shelf. From there I maneuvered to the right 45 for runway XX. On base and final I felt lower than normal and did have a sight picture for that. Uneventful landing; but I immediately checked my altimeter to field altitude and saw it was off by about 450 feet! The revelation was disconcerting. I'm certainly that it was set correctly and at field elevation at ZZZ1. 450' would be a huge change in the 30 minute flight (only about 15 miles from point to point). I should have checked it on my way in to ZZZ; but the workload was high; the approach was congested and I was trying to avoid traffic. It is possible that something in the annual caused it to drift; so I'll be having it checked before I fly again. For me; it is an impactful reminder of how important it is to regularly check the altimeter with pressure along a route.