Hi Henra,
More than a day's rest is called for. Hopefully your teacher advised you in that regard when you told her/him what was going on.
I am no doctor, but I think your muscles (not your tendons) are reacting involuntarily, which, I think, means you were playing your octaves mostly with the forearms and with relatively stiff wrist. I wouldn't worry about it being permanent, but playing through pain is a really bad idea. Graffman, Fleisher, Beroff (and countless less talented).
Next time you feel like playing octaves, it may pay off to play them from the larger muscle units of your back and upper arm, and to focus on flexible (collapsing) wrists and (if called for) knuckles. These are your shock absorbers and will eventually help you play loud and fast without getting tired or hurting yourself.
Another three thoughts on octaves, stay near the keyboard, with shaping movements so not all octaves use the same muscles in the same way, and try to be close to where you need to be if you are going between black and white keys (that is, stay close to where they meet).
Enjoy your vacation. Maybe you can read some Matthay, Breithaupt or Taubman/Golandsky while you rest.