My dog has a red, hard lump near her spay incision. What is it?

Hi there! It sounds like you're a very good friend to Athena and care very much. This looks very much like a suture reaction, or a reaction to the material of her internal stitches. The stitches we use inside the body are absorbable, and eventually break down in the tissues. Sometimes, they can cause an inflammatory reaction when this happens that looks a lot like this. If the reaction or breakdown is too soon, we risk the possibility of a hernia, etc. there is a possibility (smaller but still there) that the suture broke down a bit before it was noticed and with her exertion it just pushed its way a bit more open. If she is unbothered and has not changed in size it could probably wait until your appointment. I would recommend keeping her exercise very restricted in the meantime though-no stairs, no running outside just leash walks, etc. if you are still concerned, you can always call her doctor and see if they can get you in a bit sooner. Hope this helps!