After moving, I get headaches while hiking?

A few months ago, I moved from GA to VT. I hiked a lot back in GA without ever getting headaches or experiencing excessive tiredness/grogginess. Now that I've moved to VT, I get mild headaches an hour or so into my hikes. Then afterwards, I'm completely groggy and very tired. I'm careful to eat and drink enough, and I maintain a healthy diet. The distance and difficulty of my hikes haven't changed.

What could be causing this? Maybe just a difference in elevation?

The only way for me to relieve the grogginess after hiking is by drinking coffee. But, I don't typically drink coffee. I'm apt to think it's not healthy to just "fix" this grogginess with coffee. Or is it? Could my body just need the extra caffeine?