Good morning...first, let me say thanks for taking a rescue pup into your home and into your heart. That is a very good thing!
As far as the lump at her incision site, it could certainly be a localized reaction to the suture material, or you might even be feeling the knot of suture material used by the surgeon at the rescue.
Is she still eating and drinking ok? Is her demeanor attentive and happy?
If she still seems to be acting normal and eating/drinking ok, I don't think that this is an emergency situation, but you should still let your veterinarian take a look at her soon, especially if the lump continues to grow. She may need to be "cage rested" so that she is not too active and cause more irritation to her incision area.
Good luck with your new best friend!