As for tomorrow, I'd go buy a brace a brace and put some ice on it.
I found that making sure my wrist stays straight and my hand never moves into an awkward position helps a lot, I have my desk and seat height adjusted so my forearm is perfectly parallel with the desk when my hand is on the mouse.
Any suggestions for a good (but not overly expensive) mouse?
I used to be very fond of trackballs but never found one that combined good speed/low inertia and a useable wheel.
Good ergonomic chair, mouse etc is crucial if you are gonna do long hours of editing. And think of how you are actual sitting on the chair, how your neck is and so on.