Ergonomics question

I'm about 25 hours into my PPL and having a good time with one little complaint. Every time I'm up in the air more than an hour or so I start to get a lot of tightness and tension between my shoulders and the back of my neck, which can lead to an unpleasant tension headache the rest of the day. I don't get this normally or during other activities with a reasonably similar posture (like driving a car) and seems to be unique to flying, and my guess is something is wrong with the way I'm sitting. I can't figure out if it's because I'm sitting too high, too low, too close, too far back or what. Anyone experienced this and have any suggestions on how to fix it? Training in a C-172 for reference. Not limited to just one airplane, seems to happen regardless of which of the trainers I take up.