Some really good advice here!

Ergonomics - how the whole of your body is relating to the instrument. Posture, strap length, strap-style, size and style fo the instrument itself, what kind of chair you are sitting in (if you are sitting), tension in the neck and shoulders, and especially in the fingers, hands, and wrists. 

Little adjustments to any of these things can make a ton of difference. Experiment, notice.

Also, Make sure and "check in" with your left hand, as regards muscle tension, as you get started with a practice session. Keep checking back in throughout your session. You are looking to make the notes and chords with the least amount of muscular effort you can manage. If you notice tension in your hand, take a moment to shake it out, and then re-approach the instrument, being mindful of staying as relaxed as possible. 

As a teacher, I frequently find that a student is using way more MUSCLE than necessary to make the music happen. And this inevitably leads to strain and fatigue.
Experiment with ways to make the chords or notes sound clear, with the LEAST amount of muscular effort on the part of your hand. 

With experimentaton, will find the 'sweet spot', and hopefully be relieved of the irritation and discomfort. 
Hope this helps a little!

- Lisa