I've had aches in pinkies over the years, but they've always gone away when I rested for a while, especially if I allowed it to curve (the curve you get from letting your arm hang down--not curling). It used to happen when playing a lot of octaves. The natural curve is what really helped me, so I concluded that I had been staying in extension too long in those pieces. If you have been resting and are still in pain, I concur that you show it to someone, and see if you can make enough fuss to get a hand specialist, because family doctors will often tell you to ice and rest things, but they don't necessarily know what's wrong with it. I injured a tendon recently (not from playing, but then I couldn't play), and my doctor told me to just give it rest. Then I saw a hand surgeon who said it wasn't damaged, just temporary, and sent me to physical therapist-- where I learned that simply resting it would allow any scar tissue from the injury to get thick and stiff, and shorten the tendon a bit. I stretch it in a special way and massage it every day to keep it long and flexible. So you might want to get it checked out.
I have small hands and my hand is pretty much fully extended at the octave. In order to keep them supple in playing lots of octaves, I don't "hold" my hand open when playing a succession of octaves. I let the hand spring back to its natural-hanging-curve position. Well, that's my starting point. I start practicing the octaves slowly and let the hand spring back all the way into curve. When I get faster, I don't have time to spring back as far, and at tempo my fingers don't actually move out of position--or just barely--but I still allow the tiny moment of release after each octave in the same way. I think this "spring" might be something that people with big hands don't have to worry about as much, since they are not at full extension when playing octaves. But I am. I'm not stretching or reaching, but it is the limit, so I built these ongoing releases into my technique. I don't get the aching pinkies anymore, even with a lot of octave work, so this has really helped me. Keep us posted!! Best of luck.