I experienced this too and the most common cause is that as a beginner, you did not position your left hand properly around the neck. Most beginner actually grip the neck with their palm such that the thumb is at the level above the neck. This is fine when you play simple chords and strum along stuff (which rightly so cos most beginner start by doing this) but as you learn scales and stuff, it's important to have good left hand positioning. The correct position would be that the ball of your thumb should be placed behind the neck, your palm should be curved under the neck and your fingers slightly flexed.
Now that your hand is curved and levered forward, you realise that the whole fretboard is within easy reach but your wrist is bent more. Relax the wrist and it should not hurt so much. Maintain this left hand position first while sitting, then standing with guitar strap shorter and guitar close to your body and guitar neck angled upwards, then to standing with guitar strap longer, guitar at waist level and neck of guitar parallel to ground.
Then.... you can extend strap all the way and play like Slash or Dan Donegan (is that his name? Guitarist of Disturbed) Ha ha!
Good luck
Arsony