Perhaps try to avoid tension in your forearms and wrists.
I've also gotten pain along the outer length of my arms, which I'm certain is lactic acid build-up. I believe this is like exercise at the gym, so with a good diet and enough sleep you'll find that the pain lessens and eventually disappears coupled with a marked increase in your endurance.
Not that I want to write off this possibility outright, but I'm really not convinced. Lactic acid build up is easily removed by warming down. I don't believe this kind of pain is removed by light exercise of the 5th finger. Also, lactic acid is produced by anaerobic respiration. I don't think anyone works so hard that they cannot involve enough oxygen to use aerobic respiration. The lungs certainly wouldn't have a problem getting enough oxygen into the blood. We're almost certainly looking at inherent muscle fatigue, rather than lactic acid. Arguably, this is one of the aches that you sometimes have to go through to develop a good fifth finger, although you have to be very careful not to push yourself too hard.
Dear total anonymity, you may well have a poor technique. Nothing can be done about that in two weeks.
What a rude and needlessly negative post! What is that worthless statement supposed to offer the poster?
The insight that there are no quick fixes in piano technique.
That does not require suggesting that somebody's technique is probably "poor" and implying that they are basically screwed.
Your words not mine.
"probably" or "may" have "poor technique" is scarcely of consequence.
There's a big difference between 'may' and 'probably'. Your straw man technique just doesn't wash - and try to refrain from trashing yet another thread.
As for "trashing the thread", I have provided some constructive advice about differentiating between practising and performance